Optical sheet, light-emitting device, and method for manufacturing optical sheet

ABSTRACT

A surface of a transparent substrate  5  which is adjacent to a light emitting body has a surface structure  13  that is defined by dividing the surface into a plurality of linear segments of width w which are inclined so as to extend in a specific direction in a plane of the surface and dividing the linear segments into minute regions aligned along the longitudinal direction of the linear segments such that the largest inscribed circle of the minute regions has a diameter from 0.2 μm to 1.5 μm. Each of the minute regions is formed by a raised or recessed portion formed in the surface of the transparent substrate  5 . The proportion of the raised portions and the proportion of the recessed portions, P and 1−P, are determined such that P is in the range of 0.4 to 0.98.

TECHNICAL FIELD

The present invention relates to a transparent sheet for use with alight emitting body with one of the surfaces of the sheet being adjacentto the light emitting body, a light emitting device, and methods offabricating the same.

BACKGROUND ART

There are conventional techniques disclosed in Patent Documents 1 and 2,for example.

FIG. 1 shows a cross-sectional structure of a light emitting deviceemploying a common organic electroluminescence element (organic ELelement) and propagation of light. An electrode 102, a light emittinglayer 103, and a transparent electrode 104 are stacked in this order ona substrate 101, and a transparent substrate 105 is provided on thetransparent electrode 104. When a voltage is applied between theelectrode 102 and the transparent electrode 104, light is radiated froma point S in the light emitting layer 103. The light enters thetransparent electrode 104 directly or after being reflected by theelectrode 102, and is then transmitted through the transparent electrode104. The light transmitted through the transparent electrode 104impinges on a surface of the transparent substrate 105 at point P at anincidence angle θ from the normal to the surface. At the point P, thelight is refracted to be emitted into an air layer 106.

When the incidence angle θ exceeds the critical angle θ_(c)=sin⁻¹(1/n′₁)where n′₁ is the refractive index of the transparent substrate 105,total reflection occurs. For example, a light ray that is incident onthe surface of the transparent substrate 105 at point Q at an anglegreater than or equal to θ_(c) is totally reflected without beingemitted into the air layer 106.

FIGS. 2( a) and 2(b) are diagrams for illustrating the light extractionefficiency of the light emitting device on the assumption that thetransparent substrate 105 has a multilayer structure. In FIG. 2( a), theformula shown below holds according to Snell's law:n′ _(k)×sin θ′_(k) =n′ _(k-1)×sin θ_(k-1) = . . . =n′ ₁×sin θ′₁ =n ₀×sinθ₀  (Formula 1)where n′_(k) is the refractive index of the light emitting layer 103; n₀is the refractive index of the air layer 106; n′_(k-1), n′_(k-2), . . ., and n′₁ are the refractive indices of a plurality of interveningtransparent layers between the light emitting layer 103 and the airlayer 106 in order of distance from the light emitting layer 103,closest first; θ′_(k) is the propagation direction of a light rayradiated from the point S in the light emitting layer 3 (the angle fromthe normal to a refracting surface); and θ′_(k-1), θ′_(k-2), . . . ,θ′₁, and θ₀ are the angles of refraction at the refracting surfaces inorder of distance from the light emitting layer 103, closest first.

Therefore, the formula shown below holds:sin θ′_(k)=sin θ₀ ×n ₀ /n′ _(k)  (Formula 2)

Thus, Formula 2 is basically identical with Snell's law under thecondition that the light emitting layer 103 is in direct contacts withthe air layer 106. Formula 2 means that total reflection occurs whenθ′_(k)≧θ_(c)=sin⁻¹(n₀/n′_(k)) irrespective of the refractive indices ofthe intervening transparent layers.

FIG. 2( b) schematically shows the range of light which can be extractedfrom the light emitting layer 103. The light which can be extracted isincluded in the extent of a pair of cones 107 and 107′ whose vertexesare at the light radiation point S. The vertex angle of each of thecones 107 and 107′ is twice the critical angle θ_(c). The center axes ofthe cones 107 and 107′ are on the z-axis that is normal to therefracting surface. Assuming that the light is radiated from the point Swith equal intensities in all directions and that the transmittance oflight which is incident on the refracting surface at an incidence angleequal to or smaller than the critical angle is 100%, the extractionefficiency η from the light emitting layer 103 is equal to the ratio ofpart of the surface area of the sphere 108 corresponding to the circularbases of the cones 107 and 107′ to the entire surface area of the sphere108, and is expressed by the formula shown below:η=1−cos θ_(c)  (Formula 3)

Note that the actual extraction efficiency η is smaller than 1−cos θ_(c)because the transmittance for the incidence angles equal to or smallerthan the critical angle does not reach 100%. The total efficiency of thelight emitting element is equal to a value obtained by multiplying theabove-described extraction efficiency η by the light emission efficiencyof the light emitting layer.

Patent Document 1 discloses an organic EL element, in the context of theabove-described mechanism, which is based on the concept that adiffraction grating is formed in a substrate interface, an internalsurface, or a reflecting surface to change the incidence angle of lighton a light extraction surface such that the light extraction efficiencyis improved with the view of preventing total reflection of lightpropagating from the transparent substrate to the ambient air at thetransparent substrate surface.

Patent Document 2 describes providing a plurality of protrusions overthe surface of a transparent substrate of an organic EL element suchthat reflection of light at the interface between the transparentsubstrate and the air layer can be prevented, for the purpose ofproviding a planar light emitting device with excellent light extractionefficiency.

CITATION LIST Patent Literature

-   Patent Document 1: Japanese Laid-Open Patent Publication No.    11-283751-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2005-276581

SUMMARY OF INVENTION Technical Problem

However, the above-described conventional light emitting devices haveproblems which will be described below.

In a conventional light emitting device which employs the organic ELelement shown in FIG. 1, the maximum value of the light extractionefficiency η from the light emitting layer 103 does not exceed 1−cosθ_(c). If the refractive index of the light emitting layer 103 isdetermined, the maximum value of the light extraction efficiency isuniquely limited. For example, when n₀=1.0 and n′_(k)=1.457 in Formula2, the critical angle θ_(c)=sin⁻¹(n₀/n′_(k))=43.34°, and the maximumvalue of the light extraction efficiency is as small as about 1−cosθ_(c)=0.273. When n′_(k)=1.70, the maximum value of the light extractionefficiency is even smaller, e.g., about 0.191.

The technique disclosed in Patent Document 1 enables extraction of lightwhich would otherwise be totally reflected, although the opposite casemay occur. Assuming that there is no diffraction grating layer, a lightray emitted from a point in the light emitting layer may impinge on therefracting surface (emission surface) of the transparent substrate at anincidence angle smaller than the critical angle and then be transmittedand refracted. When there is a diffraction grating layer which diffractsthe light ray, the incidence angle on the refracting surface may exceedthe critical angle such that the light ray can be totally reflected.This means that the technique disclosed in Patent Document 1 does notnecessarily ensure the improvement of the light extraction efficiency.In the technique disclosed in Patent Document 1, diffracted light rayswhose directions are shifted by equal amounts are derived from each oneof the light rays. Light including such diffracted light rays has alight intensity distribution which varies depending on the direction,and the predetermined amount of shift width depends on the wavelength ofemitted light. Thus, color imbalance occurs depending on the direction.

In the light emitting device disclosed in Patent Document 1, ambientlight (incoming from the air layer side) is regularly reflected by thesurface of the transparent substrate, and this reflection causes adisturbance with the light extracted from the light emitting layer(resulting in, so-called, “ambient light reflection”). Therefore, thesurface of the transparent substrate needs an optical treatment, such asan antireflection film or the like, which increases the manufacturingcost.

The light emitting device disclosed in Patent Document 2 is directed toantireflection at the refracting surface. The structure of PatentDocument 2 only improves the light extraction efficiency by about 10% to20%.

The present invention was conceived in view of the above circumstances.One of the objects of the present invention is to provide an opticalsheet for use in a light emitting device, a light emitting device, andmethods of fabricating the same, which enable light which is incident onthe transparent substrate at an incidence angle equal to or greater thanthe critical angle to be emitted out such that the light extractionefficiency can be greatly improved, and which prevent ambient lightreflection as well as variation in light intensity distribution andcolor imbalance which would occur depending on the direction.

Solution to Problem

The first sheet of the present invention is a transparent sheet for usewith a light emitting body with one of surfaces of the transparent sheetbeing adjacent to the light emitting body, wherein the other surface ofthe transparent sheet includes a plurality of minute regions δ, alargest inscribed circle of the minute regions δ having a diameter from0.2 μm to 1.5 μm, one of the minute regions δ being adjoined by andsurrounded by some other ones of the minute regions δ, the plurality ofminute regions δ include a plurality of minute regions δ₁ which arerandomly selected from the plurality of minute regions δ so as toconstitute 40% to 98% of the minute regions δ and a plurality of minuteregions δ₂ which constitute the remaining portion of the minute regionsδ, the minute regions δ₁ are protruding above the other surface to aheight of d/2 relative to a predetermined reference plane parallel tothe other surface, the minute regions δ₂ are receding below the othersurface to a depth of d/2 relative to the predetermined reference plane,the predetermined reference plane is equidistant from the minute regionsδ₁ and the minute regions δ₂ in terms of a direction perpendicular tothe other surface, d is from 0.2 μm to 1.4 μm, the plurality of minuteregions δ have a shape of a polygon in a plane of the other surface, theplurality of minute regions δ are arranged in the plane of the othersurface along a first direction that is parallel to one side of thepolygon, and ones of the plurality of minute regions δ adjoining eachother along the first direction are at positions which are coincidentwith each other in terms of a second direction that is perpendicular tothe first direction, and a pair of minute regions adjoining each otheralong the second direction are at positions which are different fromeach other in terms of the first direction.

The second sheet of the present invention is a transparent sheet for usewith a light emitting body with one of surfaces of the transparent sheetbeing adjacent to the light emitting body, wherein the other surface ofthe transparent sheet includes a plurality of minute regions δ, alargest inscribed circle of the minute regions δ having a diameter from0.2 μm to 1.5 μm, one of the minute regions δ being adjoined by andsurrounded by some other ones of the minute regions δ, respective onesof the plurality of minute regions δ have random heights within a rangeof 0 to d/2 relative to a predetermined reference plane parallel to theother surface or have random depths within a range of 0 to d/2 relativeto the predetermined reference plane, the predetermined reference planeis equidistant from a highest one of the minute regions δ and a lowestone of the minute regions δ in terms of a direction perpendicular to theother surface, d is from 0.2 μm to 1.4 μm, the plurality of minuteregions δ have a shape of a polygon in a plane of the other surface, theplurality of minute regions δ are arranged in the plane of the othersurface along a first direction that is parallel to one side of thepolygon, and ones of the plurality of minute regions δ adjoining eachother along the first direction are at positions which are coincidentwith each other in terms of a second direction that is perpendicular tothe first direction, and a pair of minute regions adjoining each otheralong the second direction are at positions which are different fromeach other in terms of the first direction.

The third sheet of the present invention is a transparent sheet for usewith a light emitting body with one of surfaces of the transparent sheetbeing adjacent to the light emitting body, wherein the other surface ofthe transparent sheet includes a plurality of minute regions δ, alargest inscribed circle of the minute regions δ having a diameter from0.4 μm to 1.0 μm, one of the minute regions δ being adjoined by andsurrounded by some other ones of the minute regions δ, the plurality ofminute regions δ include a plurality of minute regions δ₁ and aplurality of remaining minute regions δ₂, the minute regions δ₁ and theminute regions δ₂ are configured to produce a phase difference of 180°between part of light perpendicularly impinging on the one surface whichis transmitted through the minute regions δ₁ and another part of thelight perpendicularly impinging on the one surface which is transmittedthrough the minute regions δ₂, the plurality of minute regions δ have ashape of a polygon in a plane of the other surface, the plurality ofminute regions δ are arranged in the plane of the other surface along afirst direction that is parallel to one side of the polygon, and ones ofthe plurality of minute regions δ adjoining each other along the firstdirection are at positions which are coincident with each other in termsof a second direction that is perpendicular to the first direction, anda pair of minute regions adjoining each other along the second directionare at positions which are different from each other in terms of thefirst direction.

In one embodiment, the minute regions δ are polygonal and congruent witheach other.

In one embodiment, the plurality of minute regions δ have a shape of arectangle or right square in the plane of the other surface, theplurality of minute regions δ are aligned along the first direction thatis parallel to one side of the rectangle or right square, ones of theplurality of minute regions δ adjoining each other along the firstdirection are at positions which are coincident with each other in termsof a second direction that is perpendicular to the first direction, anda pair of minute regions adjoining each other along the second directionare at positions which are different from each other in terms of thefirst direction.

In one embodiment, the first direction and the second direction areinclined from an edge of the sheet.

The first light emitting device of the present invention is a lightemitting device including a light emitting body and a transparentprotection layer provided on a light emitting surface of the lightemitting body, wherein the transparent protection layer has a surfacewhich adjoins the light emitting surface and a surface opposite to theadjoining surface, the opposite surface including a plurality of minuteregions δ, a largest inscribed circle of the minute regions δ having adiameter from 0.2 μm to 1.5 μm, one of the minute regions δ beingadjoined by and surrounded by some other ones of the minute regions δ,the plurality of minute regions δ include a plurality of minute regionsδ₁ which are randomly selected from the plurality of minute regions δ soas to constitute 40% to 98% of the minute regions δ and a plurality ofminute regions δ₂ which constitute the remaining portion of the minuteregions δ, the minute regions δ₁ are protruding above the other surfaceto a height of d/2 relative to a predetermined reference plane parallelto the other surface, the minute regions δ₂ are receding below the othersurface to a depth of d/2 relative to the predetermined reference plane,the predetermined reference plane is equidistant from the minute regionsδ₁ and the minute regions δ₂ in terms of a direction perpendicular tothe other surface, the light emitting body is configured to emit lightwhose center wavelength of an emission spectrum is λ,λ/6(n₁−n₀)<d<λ/(n₁−n₀) holds where n₁ is a refractive index of theprotection layer and n₀ is a refractive index of a medium with which theprotection layer is in contact at the opposite surface, n₀ being smallerthan n₁, the plurality of minute regions δ have a shape of a polygon ina plane of the other surface, the plurality of minute regions δ arearranged in the plane of the other surface along a first direction thatis parallel to one side of the polygon, and ones of the plurality ofminute regions δ adjoining each other along the first direction are atpositions which are coincident with each other in terms of a seconddirection that is perpendicular to the first direction, and a pair ofminute regions adjoining each other along the second direction are atpositions which are different from each other in terms of the firstdirection.

The second light emitting device of the present invention is a lightemitting device including a light emitting body and a transparentprotection layer provided on a light emitting surface of the lightemitting body, wherein the transparent protection layer has a surfacewhich adjoins the light emitting surface and a surface opposite to theadjoining surface, the opposite surface including a plurality of minuteregions δ, a largest inscribed circle of the minute regions δ having adiameter from 0.2 μm to 1.5 μm, one of the minute regions δ beingadjoined by and surrounded by some other ones of the minute regions δ,respective ones of the plurality of minute regions δ have random heightswithin a range of 0 to d/2 relative to a predetermined reference planeparallel to the other surface or have random depths within a range of 0to d/2 relative to the predetermined reference plane, the predeterminedreference plane is equidistant from a highest one of the minute regionsδ and a lowest one of the minute regions δ in terms of a directionperpendicular to the other surface, the light emitting body isconfigured to emit light whose center wavelength of an emission spectrumis λ, λ/6(n₁−n₀)<d<λ/(n₁−n₀) holds where n₁ is a refractive index of theprotection layer and n₀ is a refractive index of a medium with which theprotection layer is in contact at the opposite surface, n₀ being smallerthan n₁, and the plurality of minute regions δ have a shape of a polygonin a plane of the other surface, the plurality of minute regions δ arearranged in the plane of the other surface along a first direction thatis parallel to one side of the polygon, and ones of the plurality ofminute regions δ adjoining each other along the first direction are atpositions which are coincident with each other in terms of a seconddirection that is perpendicular to the first direction, and a pair ofminute regions adjoining each other along the second direction are atpositions which are different from each other in terms of the firstdirection.

The third light emitting device of the present invention is a lightemitting device including a light emitting body and a transparentprotection layer provided on a light emitting surface of the lightemitting body, wherein the transparent protection layer has a surfacewhich adjoins the light emitting surface and a surface opposite to theadjoining surface, the opposite surface including a plurality of minuteregions δ, a largest inscribed circle of the minute regions δ having adiameter from 0.4 μm to 1.0 μm, one of the minute regions δ beingadjoined by and surrounded by some other ones of the minute regions δ,the plurality of minute regions δ include a plurality of minute regionsδ₁ and a plurality of remaining minute regions δ₂, the minute regions δ₁and the minute regions δ₂ are configured to produce a phase differenceof 180° between part of light perpendicularly impinging on the onesurface which is transmitted through the minute regions δ₁ and anotherpart of the light perpendicularly impinging on the one surface which istransmitted through the minute regions δ₂, and the plurality of minuteregions δ have a shape of a polygon in a plane of the other surface, theplurality of minute regions δ are arranged in the plane of the othersurface along a first direction that is parallel to one side of thepolygon, and ones of the plurality of minute regions δ adjoining eachother along the first direction are at positions which are coincidentwith each other in terms of a second direction that is perpendicular tothe first direction, and a pair of minute regions adjoining each otheralong the second direction are at positions which are different fromeach other in terms of the first direction.

In one embodiment, the medium is air.

In one embodiment, the medium is aerogel.

In one embodiment, n₂−n₁<0.1 holds where n₂ is a refractive index ofpart of the light emitting body from which light is radiated.

In one embodiment, the plurality of minute regions δ have a shape of arectangle or right square in the plane of the other surface, theplurality of minute regions δ are aligned along the first direction thatis parallel to one side of the rectangle or right square, ones of theplurality of minute regions δ adjoining each other along the firstdirection are at positions which are coincident with each other in termsof a second direction that is perpendicular to the first direction, anda pair of minute regions adjoining each other along the second directionare at positions which are different from each other in terms of thefirst direction.

A sheet fabrication method of the present invention is a method offabricating the first to third sheets, including pressing a side surfaceof a cylindrical mold against a sheet material such that recessedportions and raised portions corresponding to the minute regions δ areformed in one surface of the sheet material, thereby forming the sheet,wherein the side surface of the cylindrical mold has recessed portionsand raised portions corresponding to the minute regions δ, the recessedportions and the raised portions being spirally arranged around arotation axis of the cylindrical mold.

A sheet fabrication method of the present invention is a method offabricating the first to third sheets, including the steps of: pressinga side surface of a cylindrical mold against a protection layer materialsuch that recessed portions and raised portions corresponding to theminute regions δ are formed in one surface of the protection layermaterial, thereby forming the protection layer; and placing theprotection layer on an emission surface of the light emitting body suchthat a surface of the protection layer opposite to the one surface iscloser to the light emitting body, wherein the side surface of thecylindrical mold has recessed portions and raised portions correspondingto the minute regions δ, the recessed portions and the raised portionsare spirally arranged around a rotation axis of the cylindrical mold.

ADVANTAGEOUS EFFECTS OF INVENTION

With the above solutions, extraction of light rays whose incidenceangles are greater than the critical angle can be repeatedly realized,so that the light extraction efficiency can be greatly improved.Moreover, diffraction caused by the random structure leads toelimination of regularity in diffraction direction, so that ambientlight reflection as well as variation in light intensity distributionand color imbalance which would otherwise occur depending on thedirection can be prevented. Also, a sheet and a light emitting devicewhich have a simple structure, a large surface, and a pattern of highaccuracy can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional structure of an organicelectroluminescence element and propagation of light.

FIG. 2( a) shows a transparent substrate which has a multilayerstructure. FIG. 2( b) is a diagram illustrating the range of light whichcan be extracted.

FIG. 3( a) shows a step wise change of the refractive index. FIG. 3( b)shows a moderate change of the refractive index. FIG. 3( c) illustratesthe relationship between the incidence angle at a refracting surface andthe transmittance. FIG. 3( d) shows the refracting surface.

FIG. 4( a) shows a cross section of a light emitting device whichincludes a diffraction grating having a periodic structure at aninterface. FIG. 4( b) is the top view of the structure of FIG. 4( a).

FIG. 5 illustrates the direction of diffraction caused by a diffractiongrating.

FIG. 6( a) shows a cross section of a light emitting device which hasprotrusions randomly arranged on the surface. FIG. 6( b) is a top viewof the structure of FIG. 6( a).

FIGS. 7( a) to 7(h) schematically illustrate the boundary conditions ofa field of light at a refracting surface.

FIG. 8( a) shows an arrangement of a pinhole. FIG. 8( b) shows anarrangement of phase shifters.

FIG. 9 illustrates the transmittance over the incidence angle at therefracting surface over which 180°-phase shifters are randomly arranged.

FIG. 10 is an experimental explanation diagram which illustrates thetransmittance over the incidence angle at the refracting surface overwhich 180°-phase shifters are randomly arranged.

FIG. 11 shows the structure of an experimental apparatus for measuringthe transmittance relative to the incidence angle.

FIG. 12 shows a cross-sectional structure of an organicelectroluminescence element of the first embodiment and propagation oflight.

FIGS. 13( a) and 13(b) are enlarged views showing part of the surfacestructure of the first embodiment. FIG. 13( c) is a pattern diagramwhich covers a broader area.

FIGS. 14( a) to 14(d) illustrate the viewing angle dependence of lightwhich is emitted from the surface structure of the first embodiment.

FIGS. 15( a) to 15(d) illustrate the viewing angle dependence of lightwhich is emitted from the surface structure of the first embodiment.

FIG. 16 illustrates the incidence angle dependence of transmittance t ofthe surface structure of the first embodiment. FIG. 16( a) is a diagramshowing the incidence angle dependence of the transmittance in the firstlight extraction. FIG. 16( b) is a diagram showing the incidence angledependence of the transmittance in the second light extraction.

FIG. 17 is an experimental explanation diagram which illustrates theincidence angle dependence of the transmittance t of the surfacestructure of the first embodiment.

FIG. 18 illustrates the incidence angle dependence of the amount ofextracted light via the surface structure of the first embodiment. FIG.18 (a) is a diagram showing the incidence angle dependence of the amountof extracted light in the first light extraction. FIG. 18( b) is adiagram showing the incidence angle dependence of the amount ofextracted light in the second light extraction.

FIGS. 19( a) and 19(b) are diagrams which illustrate the lightextraction efficiencies of the surface structure of the firstembodiment.

FIG. 20 shows a cross section of a light emitting device which includesan adjustment layer.

FIG. 21 shows a cross section of a light emitting device which hasanother surface structure at the border with the adjustment layer.

FIG. 22( a) illustrates the light extraction efficiency of the surfacestructure of the second embodiment. FIG. 22( b) illustrates the lightextraction efficiency of the surface structure of the third embodiment.

FIG. 23 illustrates the extraction efficiency according to the secondembodiment.

FIGS. 24( a) to 24(e) illustrate how to determine the pattern of thesurface structure according to the fourth embodiment.

FIGS. 25( a) to 25(c) show the first surface structure of the seventhembodiment.

FIG. 26( a) shows a cylindrical mold for use in the present embodiment.FIGS. 26( b) and 26(c) schematically illustrate the methods oftransferring recessed and raised portions formed in the surface of thecylindrical mold 31 to a sheet 31 c based on a roll-to-roll method.

FIGS. 27( a) to 27(d) are enlarged views of part of the side surface ofthe cylindrical mold 31.

FIG. 28( a) illustrates the direction of the arrangement of the minuteregions δ formed in the side surface of the cylindrical mold 31. FIG.28( b) shows a sheet 32 in which the uneven shape of recessed and raisedportions formed in the side surface of the cylindrical mold 31 as shownin FIG. 28( a) is transferred.

FIGS. 29( a) to 29(c) show other arrangement examples of the minuteregions δ over the side surface of the cylindrical mold.

FIGS. 30( a) and 30(b) show a cross-sectional structure of an organicelectroluminescence element of another embodiment and propagation oflight.

FIG. 31 is a pattern diagram illustrating a checker-pattern surfacestructure.

FIG. 32 illustrates the incidence angle dependence of the transmittancet of the surface structure shown in FIG. 31.

FIGS. 33( a) to 33(c) illustrate how to randomly arrange protrusions.

FIGS. 34( a) and 34(b) also shows the analysis results of the viewingangle dependence of light extracted in the first light extraction, whichwas emitted from the checker-pattern surface structure.

DESCRIPTION OF EMBODIMENTS

Before the descriptions of embodiments of the present invention, theresearch history prior to the creation of the concept of the presentinvention is described with considerations for the prior art examplessuch as disclosed in, for example, Patent Document 1 and Patent Document2.

FIG. 3 illustrates the transmittance at a refracting surface (theinterface between a transparent layer surface and an air layer). Now,consider a light ray traveling from the inside of the transparent layer107 with the refractive index of 1.5 along a sheet direction andimpinging on a refracting surface 107 a of the transparent layer 107 atan incidence angle θ so that the light ray undergoes refraction towardthe air layer (refractive index: 1.0). Here, the transmittance of thislight ray relates to the polarization of the light. Usually, therefractive index distribution along the normal to the refracting surface107 a in the vicinity of the refracting surface 107 a has a steppedshape as shown in FIG. 3( a). Therefore, the P-polarization (anoscillation component whose electric field vector is parallel to thesheet of the drawing) exhibits a transmittance characteristicrepresented by the curve 108 a, and the S-polarization (an oscillationcomponent whose electric field vector is perpendicular to the sheet ofthe drawing) exhibits a transmittance characteristic represented by thecurve 108 b. They exhibit different behaviors when the incidence angleis not more than the critical angle) (=41.8°) but become zero as theincidence angle exceeds the critical angle.

On the other hand, assuming that the outermost part of the transparentlayer 107 has a multilayer structure so that the refractive indexdistribution has a tapered shape as shown in FIG. 3( b), theP-polarization exhibits a transmittance characteristic represented bythe curve 108A, and the S-polarization exhibits a transmittancecharacteristic represented by the curve 108B. Although both of thembecome zero as the critical angle is exceeded, the transmittance becomescloser to 100% when the incidence angle is equal to or smaller than thecritical angle, so that the distribution approaches to the shape of astep function with the border occurring at the critical angle. In theexample of FIG. 3( b), the calculation is based on the assumption thatthe structure has a multilayer structure of 50 stacked layers eachhaving the thickness of 0.01 μm, with the refractive index varying from1.5 to 1.0 with the intervals of 0.01. The difference between theP-polarization and the S-polarization decreases as the ramp of thechange of the refractive index along the thickness direction is moremoderate so that, as a result, as for the both polarizations, the graphof the transmittance relative to the incidence angle approaches to astep function.

To prevent total reflection, it is necessary to provide any means ofcontrolling the incidence angle of light that is incident on therefracting surface so as to be equal to or smaller than the criticalangle. We consulted Patent Document 1 for an example of such means andstudied a light emitting device that uses an organic EL element shown inFIG. 4, in which a diffraction grating 209 is provided at the interfacebetween a transparent substrate 205 and a transparent electrode 204.

As shown in FIG. 4( a), an electrode 202, a light emitting layer 203, atransparent electrode 204, and a diffraction grating layer 209 arestaked on a substrate 201 in this order, and a transparent substrate 205is provided on the diffraction grating layer 209. The diffractiongrating layer 209 has a periodic structure of raised portions andrecessed portions with the pitch of Λ in both x-direction andy-direction, at its border with the transparent substrate 205. The shapeof the raised portion may be a right square with width w as shown inFIG. 4( b). The raised portions having such a shape are in amatrix-lattice arrangement. Application of a voltage between theelectrode 202 and the transparent electrode 204 causes radiation oflight from the point S in the light emitting layer 203. This lightenters the transparent electrode 204 directly or after being reflectedby the electrode 202 and is transmitted therethrough. The light is thentransmitted through the diffraction grating layer 209 so that itundergoes diffraction. For example, assuming that a light ray 210 aemitted from the point S travels straight without being diffracted bythe diffraction grating layer 209, the light ray would impinge on arefracting surface 205 a of the transparent substrate 205 at anincidence angle equal to or greater than the critical angle and betotally reflected by the refracting surface 205 a as represented as alight ray 210 b. However, in actuality, the light ray is diffracted bythe diffraction grating layer 209 so that the incidence angle of thelight ray on the refracting surface 205 a is smaller than the criticalangle as represented as a light ray 210 c. Therefore, the light ray canbe transmitted through the refracting surface 205 a.

The direction of diffraction of light by the above-described diffractiongrating is described with reference to FIG. 5. Now, consider a light raytraveling from the inside of a transparent layer 207 of refractive indexn_(A) along the sheet direction and impinging on a refracting surface207 a of the transparent layer 207 at the point O at the incidence angleθ so that the light ray is diffracted toward a transparent layer 206 ofrefractive index n_(B). The refracting surface 207 a is provided with adiffraction grating of pitch Λ along the surface of the sheet of thedrawing. A circle 211 around the point O at the center with radius n_(A)and a circle 212 around the point O at the center with radius n_(B) aredrawn. Here, a vector originating from a point on the circle 211 anddirected to the point O at angle θ is referred to as incidence vector210 i, and the orthogonal projection vector of the incidence vector 210i onto the refracting surface 207 a (a vector extending from the foot ofthe perpendicular A to the point O) is denoted by 210I. A vector 210 roriginating from the point O and terminating at a point on the circle212 is drawn such that the orthogonal projection vector 210R of thevector 210 r is identical with the vector 210I. Now consider a vector(grating vector) originating from the foot of the perpendicular C andhaving largeness qλ/Λ. Here, q denotes the order of diffraction(integer). The drawing shows a vector 210D for q=1. A vector 210 doriginating from the point O and terminating at a point on the circle212 is also drawn such that the terminal B of the vector 210D iscoincident with the foot of the perpendicular of the vector 210 d.Considering how to draw the vectors, the directional angle φ of thevector 210 r (the angle between the vector 210 r and the normal to therefracting surface) is expressed as follows:n _(B)×sin φ=n _(A)×sin θ  (Formula 4)

This exactly represents the Snell's law. On the other hand, thedirectional angle φ′ of the vector 210 d that represents the directionof the diffracted light ray (the angle between the vector 210 d and thenormal to the refracting surface) is expressed as follows:n _(B)×sin φ′=n _(A)×sin θ−qλ/Λ  (Formula 5)

In the example of FIG. 5, the angle φ′ is defined by a negative valuebecause it passes across the z-axis (a normal to the refracting surfacewhich passes through the point O).

Thus, the diffracted light ray has a direction deviated from therefracted light ray by qλ/Λ. In FIG. 4, the light ray 210 b that isassumed as not undergoing diffraction is equivalent to a refracted lightray, and the light ray 210 c that is assumed as undergoing diffractionhas a direction deviated from the light ray 210 b by qλ/Λ so that itdoes not undergo total reflection at the refracting surface 205 a.Therefore, a light ray which would otherwise have been totally reflectedcan be extracted, so that the improvement of the light extractionefficiency may be expected as compared with an organic EL light emittingdevice which does not include a diffraction grating layer.

However, when considering a light ray 210A emitted from the point S inFIG. 4( a) on the assumption that the light ray 210A travels straightwithout undergoing diffraction by the diffraction grating layer 209, thelight ray 210A would impinge on the refracting surface 205 a of thetransparent substrate 205 at an incidence angle equal to or smaller thanthe critical angle and be refracted by the refracting surface 205 awhile being transmitted therethrough as represented as a light ray 210B.However, in actuality, it is diffracted by the diffraction grating layer209, and therefore, the incidence angle of the light on the refractingsurface 205 a is greater than the critical angle as represented as alight ray 210C, so that the light impinges on the refracting surface 205a at an incidence angle equal to or greater than the critical angle andis totally reflected. Thus, providing the diffraction grating layer 209does not necessarily ensure the improvement of the light extractionefficiency.

In a light emitting device which includes the organic EL element shownin FIG. 4, diffracted light rays whose directions are equally shifted byqλ/Λ are derived from every one of the light rays. The light includingsuch diffracted light rays has a light intensity distribution whichvaries depending on the direction, and the shift width qλ/Λ depends onthe wavelength λ of emitted light. Therefore, color imbalance occursdepending on the direction in which the light is emitted. Specifically,the color of perceived light differs depending on the viewing direction.Thus, such a device is not suitable to display applications, as a matterof course, and is also not suitable to light sources.

Next, we consulted Patent Document 2 and studied a light emitting devicethat uses an organic EL element shown in FIG. 6, in which protrusions315 are provided on the surface of a transparent substrate 305. As shownin FIG. 6( a), an electrode 302, a light emitting layer 303, atransparent electrode 304, and a transparent substrate 305 are staked ona substrate 301 in this order, and a plurality of protrusions 315 areprovided on the surface 305 a of the transparent substrate 305. Each ofthe protrusions 315 is in the shape of a quadrangular prism of width wand height h. The protrusions 315 having such a shape are placed atrandom positions over the transparent substrate surface 305 a as shownin FIG. 6( b). Here, w is in the range of 0.4 μm to 20 μm, and h is inthe range of 0.4 μm to 10 μm. Such protrusions 315 are provided in thedensity of 5000 to 1000000 protrusions/mm². Application of a voltagebetween the electrode 302 and the transparent electrode 304 causesradiation of light at the point S in the light emitting layer 303. Aradiated light ray 310 d enters the transparent electrode 304 directlyor after being reflected by the electrode 302 and is transmittedtherethrough. Part of the transmitted light is extracted outside via theprotrusions 315 as represented as a light ray 310 f. The actualprotrusions 315 can be processed by means of side etching so as to betapered to the tip end and, even without the side etching, the effectiverefractive index falls on a value around the midpoint between thetransparent substrate 305 and the air layer, so that the refractiveindex distribution can be changed constantly and moderately. Therefore,the refractive index distribution approaches to one that is shown inFIG. 3( b), and hence, reflection of light by the protrusions 315, whichis denoted by 310 e, can be partially prevented. As a result, the lightextraction efficiency can be improved. Even when the size of theprotrusions 315 is equal to or greater than the wavelength, theinterference of emitted light rays can be prevented because theprotrusions 315 are in a random arrangement.

However, when considering that, in a light emitting device which has thestructure shown in FIG. 6, the effect of the protrusions resides inantireflection as suggested in Patent Document 2, it is seen from thecomparison between the curves 108 a and 108 b and the curves 108A and108B of FIG. 3( c) that the improvement in transmittance is achievedonly for the light rays that are incident at angles equal to or smallerthan the critical angle, and the improvement of the light extractionefficiency is only 10% to 20%, in which a great improvement cannot beexpected.

After the above studies, the present inventors continued to examine howto decrease the amount of total reflection by the refracting surface andhow to increase the amount of light which can be extracted. At thebeginning of the continued examination, we studied the boundaryconditions of light at the refracting surface.

FIG. 7 schematically illustrates the boundary conditions of a field oflight at the refracting surface, in which a light beam of width Wimpinging on the refracting surface T is considered. According to theMaxwell equations, the integral of an electric field vector or amagnetic field vector along a closed path A that is traced so as totraverse the refracting surface T is zero. Note that the premisesassumed herein are that there is not a charge or a light source in anarea inside the closed path and that the intensity and phase of theelectric or magnetic field vector along the refracting surface T arecontinuous.

When width W is sufficiently large as shown in FIG. 7( a), width t thatis perpendicular to the refracting surface can be negligibly small ascompared with width s that is taken along the refracting surface. Of thecontour integral, only components along the refracting surface areremaining. This relationship requires that the electric or magneticfield vector is continuous so as to extend through the refractingsurface. One that is derived by utilizing this continuity relationshipis the Fresnel's formula, by which the laws of reflection andrefraction, the phenomenon of total reflection, etc., are completelyelucidated.

As shown in FIG. 7( b), when the width W of the light is several tentimes smaller than the wavelength, the width t is nonnegligible. Here,when the contour integral A is divided into B and C (see FIG. 7( c)),the contour integral B is included in the light beam and is thereforezero. As for the remaining contour integral C, the electric or magneticfield vector outside the light beam is zero, so that only the integralvalue of the path PQ that is within the light beam is remaining (seeFIG. 7( d)). Therefore, the contour integral C is not zero so that, bycalculation, it is equivalent to a condition where light is radiatedinside the closed path. When the width W of the light beam is as smallas about 1/10 of the wavelength, the contour integrals C and C′ becomecloser to each other as shown in FIG. 7( e) so that the paths PQ andQ′P′ overlap with each other. Accordingly, the contour integral of Cplus C′ is zero, so that light cannot be radiated within the closedpath.

On the other hand, on the assumption that light beams between which thephase difference is n occur side by side along the refracting surface asshown in FIG. 7( f), the contour integral A which extends over theselight beams is now considered. In this case also, when the width W ofthe light is several ten times smaller than the wavelength, the width tis nonnegligible. Here, when the contour integral A is divided into B,C, and B′ (see FIG. 7( g)), the contour integrals B and B′ are includedin the light beams so that they are zero. As for the remaining contourintegral C, the components along the refracting surface are negligible,so that only the integral value of the paths PQ and Q′P′ that extendalong the boundary between the two light beams is remaining (see FIG. 7(h)). Since the integral along the path Q′P′ in a field in which thephase of the light beam is n is equal to the integral along the pathP′Q′ of a field in which the phase of the light beam is 0, the contourintegral C is twice the integral along the path PQ so that, bycalculation, it is equivalent to a condition where light is radiatedinside the closed path. Therefore, not only in the case of a narrowlight beam but also in the case where light beams in different phasesoccur side by side with a narrow gap therebetween, light occurs near theboundary of the width. (This phenomenon is not actual emission of lightbut behavior of light which is effectively equivalent to emission oflight. This is similar to a phenomenon called “boundary diffraction” byYoung prior to the establishment of the diffraction theory, and istherefore referred to as a “boundary diffraction effect.”)

If radiation of light occurs on the refracting surface T, the lightwould propagate through both media on the opposite sides of therefracting surface irrespective of the incidence conditions on therefracting surface T. That is, even in the case of a light ray that isincident at an angle equal to or greater than the critical angle, it isestimated that transmitted light rays would occur without being totallyreflected so long as the structure is configured by calculation suchthat radiation of light occurs on the refracting surface. Based on theresults of such examinations, the present inventors studied thestructure of the refracting surface, as will be described below, withthe view of actually producing a phenomenon that light is transmittedthrough the refracting surface even when the incidence angle of thelight on the refracting surface exceeds the critical angle.

Two examples are shown in FIG. 8, in which a large boundary diffractioneffect is obtained. (a) A pinhole is provided at the boundary between atransparent substrate placed on a light emitting body and the air layerwhile light is blocked in the other area so that pinhole light isobtained (light exists only in a white square box). (b) 180°-phaseshifters 18 are arranged at random in the boxes of a chessboard-likegrid, each box having width w. First, the pinhole example was examined,but actual light extraction via the pinhole could not be achieved. Theexample of phase shifters in a random arrangement was examined which wasexpected to achieve an equal light extraction characteristic to that ofthe pinhole.

FIG. 9 illustrates the incidence angle dependence of the transmittance tat the refracting surface in the structure shown in FIG. 8. Here, alight beam at the wavelength of 0.635 μm, the light amount of which is 1in a transparent substrate with the refractive index of 1.457, impingeson the boundary between the transparent substrate and the air layer atthe incidence angle θ (the angle between the light beam and the normalto the refracting surface). The curves show how much of the lightimpinging on the refracting surface for the first time is emitted intothe air, with the parameter of width w (w=0.1, 0.2, 0.4, 0.6, 0.8, 1.0,2.0, 4.0, 20.0 (μm)). (The 180°-phase shifters 18 are used insteadbecause the pinhole light and the 180°-phase shifters exhibit exactlyequal characteristics.) As for the characteristic for w=20 μm which isapproximate to that obtained in the conditions of FIG. 7( a), thetransmittance is approximately zero when the critical angle) (43.34° isexceeded. When w decreases to 0.4 to 1.0 μm, the transmittance is largeeven when the critical angle is exceeded, due to the boundarydiffraction effect which has been described with reference to FIGS. 7(d) and 7(h). When w is further decreased (w=0.1 μm, 0.2 μm), thetransmittance becomes closer to 0 at every incidence angle as describedwith reference to FIG. 7( e). Note that the analysis result of FIG. 9 isbased on the Helmholtz's wave equation (so called “scalar waveequation”), and therefore, no difference occurs between theP-polarization and the S-polarization.)

FIG. 10 shows the experimental results which illustrate the incidenceangle dependence of the transmittance t of incident P-polarization forthe first time. Since fabrication of minute phase shifters 18 isdifficult in actual manufacturing, a mask in which portions of phase 0°transmit light while portions of phase 180° are covered with a lightblocking film (Cr film) is used instead in experiments. (The mask isformed by randomly arranging light blocking films in the boxes of achessboard-like grid, each box having width w. This mask is equivalentto a random arrangement of pinhole light spots.). In the actuallyfabricated mask patterns, width w was 0.6 μm, 0.8 μm, 1.0 μm, 2.0 μm,and 5.0 μm. The apparatus used in the experiments was composed of asemiconductor laser (wavelength: 0.635 μm), a triangular prism 58 (BK7),a mask substrate 59 (synthetic quartz, refractive index: 1.457, with amask pattern formed on the back surface), a light collecting lens system50, and a photodetector 51 as shown in FIG. 11. The triangular prism wastightly placed on the surface of the mask substrate with theintervention of a matching solution 52 of refractive index 1.51. A laserlight beam was provided from the triangular prism side while thedirectional angle was measured, and transmitted light leaking from theback surface side was collected by the light collecting lens system 50.The amount of the transmitted light was measured by the photodetector51. In the case of the mask, the light was blocked by the light blockingfilm portions which are equal to ½ of the total area, and as such, theamount of the transmitted light was ½ of that transmitted when the phaseshifters were used. Thus, the transmittance t was normalized with theamount of light impinging on the portions that are not provided with alight blocking film (½ of the total light amount). The experimentalresults exhibit a good agreement with the analysis results shown in FIG.9. It is understood that the transmittance is large even when thecritical angle) (43.34° is exceeded, and that this tendency grows as wbecomes smaller.

Based on the above results, the present inventors continued studies tofinally arrive at a novel light emitting device in which totalreflection is prevented so that the light extraction efficiency can begreatly improved.

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings. In the drawings that will bereferred to in the following sections, components that havesubstantially the same functions are denoted by the same referencenumerals for the sake of simplicity of description.

First Embodiment

The first embodiment is described with reference to FIGS. 12 to 19 andFIG. 27.

FIG. 12 shows a cross-sectional structure of a light emitting devicewhich includes an organic EL element according to the first embodimentand propagation of light. An electrode 2, a light emitting layer 3, atransparent electrode 4 are stacked on a substrate 1 in this order, anda transparent substrate (transparent protection layer) 5 is provided onthe transparent electrode 4. The substrate 1, the electrode 2, the lightemitting layer 3, and the transparent electrode 4 constitute a lightemitting body. A surface of the transparent substrate 5 is provided witha surface structure 13 which is divided into minute regions and whichhas fine recessed and raised portions.

Application of a voltage between the electrode 2 and the transparentelectrode 4 causes radiation of light at the point S in the lightemitting layer 3. This light enters the transparent electrode 4 directlyor after being reflected by the electrode 2 and is transmittedtherethrough. The transmitted light impinges on the surface structure 13of the surface of the transparent substrate 5 at point P at theincidence angle θ relative to the normal to the surface. At this point,the light is diffracted by the surface structure 13 to be emitted intothe air layer 6.

Total reflection must occur when the incidence angle θ is greater thanthe critical angle θc=sin⁻¹(n₀/n₁) where n₀ is the refractive index ofthe air layer 6 and n₁ is the refractive index of the transparentsubstrate 5. However, there is the surface structure 13 in the surfaceof the transparent substrate 5, and therefore, light impinging on pointQ at an incidence angle equal to or greater than the critical angleθ_(c) is diffracted without being totally reflected, so that thediffracted light is emitted into the air layer 6 (first lightextraction). Note that, at point Q, part of the light is reflected. Thereflected component is then reflected by the electrode 2 and againimpinges on the surface structure 13 at point R. Part of the lightimpinging on the surface structure 13 at point R is emitted into the airlayer 6 (second light extraction) while the remaining part of the lightis reflected. This process is repeated endlessly.

Now, consider a light emitting device which includes a conventionalorganic EL element that does not have the surface structure 13. Lighttransmitted through the transparent substrate and impinging on theinterface between the transparent substrate and the air layer at anincidence angle equal to or greater than the critical angle is totallyreflected. Even if the totally reflected light is then reflected by theelectrode, the light reflected by the electrode impinges on theinterface between the transparent substrate and the air layer again atan incidence angle equal to or greater than the critical angle, so thatthe second and subsequent light extractions do not occur. In this point,the example considered herein is different from the present embodiment.

Hereinafter, the surface structure 13, which is a feature of the presentembodiment, is described in detail.

FIG. 13 shows two examples of the pattern of the surface structure 13 ofthe first embodiment. The left part of the FIGS. 13( a) and 13(b) showstop views, and the right part shows cross-sectional views taken alongline A-A of the top views. The surface structure 13 shown in FIG. 13( a)is formed by dividing the surface of the transparent substrate 5 intoboxes of a chessboard-like grid (square minute regions δ), each of whichhas the width w (referred to as “boundary width”), without leaving anygap therebetween and randomly allocating raised portions (which aredenoted by “13 a” in the drawing (minute regions δ₁), dotted boxes) orrecessed portions which are defined relative to the raised portions(which are denoted by “13 b” in the drawing (minute regions δ₂), whiteboxes) to respective ones of the boxes (minute regions δ) such that theproportion of the raised portions or recessed portions is 50% within atwo dimensional field. FIG. 13( c) shows an example where w=0.4 μm(black boxes correspond to the raised portions, and white boxescorrespond to the recessed portions). On the other hand, the surfacestructure 13 shown in FIG. 13( b) is formed by dividing the surface ofthe transparent substrate 5 into square minute regions δ of the width wwithout leaving any gap therebetween as in FIG. 13( a). Note that, inthe grid shown in FIG. 13( b), every other column of the minute regionsδ is shifted, whereas the minute regions δ shown in FIG. 13( a) are in aregular chessboard-like arrangement.

Specifically, as shown in FIG. 13( b), the minute regions δ are arrangedin the plane of the surface of the transparent substrate 5 along thedirection A that is parallel to one side a of the square minute regionsδ and along the direction B that is perpendicular to the direction A.Two minute regions 13 a and 13 b′ adjoining along the direction A are atpositions which are coincident with each other in terms of the Bdirection (the sides of the minute regions 13 a and 13 b′ extending inthe A direction are at positions which are coincident with each other interms of the B direction). On the other hand, two minute regions 13 aand 13 b adjoining along the direction B are at positions which aredifferent from each other in terms of the A direction (the sides of theminute regions 13 a and 13 b extending along the B direction are atpositions which are different from each other in terms of the Adirection).

In the surface structure 13 shown in FIG. 13( a), the minute regions δare randomly arranged in a two-dimensional field, whereas in the surfacestructure 13 shown in FIG. 13( b), the minute regions δ are arrangedaccording to a different rule. The surface structure 13 shown in FIGS.13( a) and 13(b) are patterned by cutting linear segments of anappropriate length from the linear segment of the minute regions δ(width w×length w) that are in a one-dimensionally random arrangement asshown in FIG. 27( a) and arranging the linear segments along a specificdirection in the plane. For example, in the patterns shown in FIGS. 27(c) and 27(d), linear segments of four aligned minute regions δ are cutout, and these linear segments are arranged side by side along thehorizontal direction. The length of the linear segments is arbitrary.The length of the linear segment does not need to be a multiple of w.Along the vertical direction, the minute regions δ are randomlyarranged, whereas the arrangement of the minute regions δ along thehorizontal direction does not need to be defined according to any rule.

Thus, only the arrangement of the minute regions along a specificdirection in the plane of the sheet is random, while the arrangement ofthe minute regions along a direction perpendicular to the specificdirection (horizontal direction) and the rule for the arrangement alongthe perpendicular direction do not matter. Even with such a pattern, therandomness along the horizontal direction can be maintained.Specifically, although the patterns of the sheet shown in FIGS. 13( a)and 13(b) appear different, the total number of boundaries which areobtained from the uneven shape of recessed and raised portions of theminute regions δ scarcely changes, and the optical characteristicsobtained are basically the same, even when a positional deviation whichis not greater than the minimum unit w occurs.

Note that, in the pattern of the sheet shown in FIG. 13( b), the minuteregions are not arranged in straight lines along the horizontaldirection, and therefore, the pattern of the sheet shown in FIG. 13( b)has improved randomness in the arrangement of the minute regions ascompared with the pattern of the sheet shown in FIG. 13( a).

In FIGS. 13( a) and 13(b), the height of the raised portions that formthe minute regions δ from the bottom of the recessed portions is d.Specifically, one of the minute regions δ is adjoined by and surroundedby some other ones of the minute regions δ. The minute regions δ₁protrude above the surface of the transparent substrate 5 to a levelhigher than the minute regions δ₂. Considering that there is a referenceplane which is parallel to the surface of the transparent substrate 5and which is equidistant from the minute regions δ₁ and the minuteregions δ₂ in terms of a direction perpendicular to the surface of thetransparent substrate 5, the minute regions δ₁ are protruding above thereference plane by d/2, and the minute regions δ₂ are receding below thereference plane by d/2. This can be explained in a different way.Consider that a surface of the transparent substrate 5 at its borderwith the air 6 has a plurality of recesses (white boxes) while the otherportions of the surface than the recesses are coplanar, and the depthsof the recesses are substantially equal to d, the bottom surfaces of therecesses being referred to as the first reference plane. In this case,the first reference plane includes a plurality of divisional minuteregions δ having equal areas, each of which is 1.5×1.5 μm² or smaller.The shape of the bottom surface of the recess is identical with a shapeof two or more minute regions δ joined together or a shape of only oneminute region δ. The recesses are randomly arranged over the firstreference plane. Note that the first reference plane is different fromthe above-described reference plane.

The formation of the surface structure 13 may be realized by preparing amold which has recessed portions and raised portions formed by means ofetching, transferring the shape of the mold to a resin sheet bypressing, and attaching the resultant sheet onto the transparentelectrode 4 with the intervention of the transparent substrate 5 whichfunctions as an adhesive layer. In this case, the transparent substrate5 is a transparent sheet. Alternatively, the surface structure 13 may berealized by directly forming recessed portions and raised portions bymeans of etching or the like in a surface of a sheet or in the surfaceof the transparent substrate 5 which has been formed as a protectionlayer.

Light diffracted by such a random pattern has random propagationdirections. Therefore, variation in light intensity distribution andcolor imbalance which would occur depending on the direction in thelight emitting device described in Patent Document 1 would not occur inthe present embodiment. Light incoming from the external environment(air layer side) is reflected by the surface structure 13 formed in thesurface of the transparent substrate 5. This reflected light isdiffracted in random directions, so that an ambient image is notreflected in the surface. Therefore, an optical treatment, such as anantireflection film or the like, is not necessary and, accordingly, themanufacturing cost can be decreased. FIG. 14 and FIG. 15 show theresults of analysis of the viewing angle dependence of the light whichis extracted from, i.e., emitted from the surface structure of the firstembodiment in the first light extraction (first extracted light). Here,the height difference is d=0.7 μm, and the wavelength λ and the boundarywidth w are parameters. In FIG. 14( a), these parameters are λ=0.450 μmand w=0.5 μm; in FIG. 14( b), λ=0.635 μm and w=0.5 μm; in FIG. 14( c),λ=0.450 μm and w=1.0 μm; in FIG. 14( d), λ=0.635 μm and w=1.0 μm; inFIG. 15( a), λ=0.450 μm and w=1.5 μm; in FIG. 15( b), λ=0.635 μm andw=1.5 μm; in FIG. 15( c), λ=0.450 μm and w=2.0 μm; and in FIG. 15( d),λ=0.635 μm and w=2.0 μm. A vector extending from the origin to a pointon the curve represents the light intensity and the emission directionof emitted light. The length of the vector corresponds to the lightintensity, and the direction of the vector corresponds to the emissiondirection. The vertical axis corresponds to the direction of an axisnormal to the plane (plane-normal axis), and the horizontal axiscorresponds to the direction of an axis in the plan (in-plane axis). Thesolid line represents the characteristic in a cross section in which thein-plane axis extends along the x-axis or y-axis of FIG. 13( b)(longitudes 0° and 90°). The broken line represents the characteristicin a cross section in which the in-plane axis extends along the line ofy=x or y=−x (longitudes 45° and 135°). (The result for the longitude 90°is equal to that for the longitude 0°, and the result for the longitude135° is equal to that for the longitude 45°. Therefore, the results forthese longitudes are not shown.) For the boundary widths w=0.5 μm and1.0 μm, both the solid line and the broken line exhibit moderatevariations relative to the declination (latitude), i.e., exhibit a smalldifference in intensity that may be caused due to the parallax, andexhibit an agreement with each other. As w is increased to w=2.0 μm, thevariation in intensity relative to the declination increases around theplane-normal direction. When λ=0.450 μm, the separation of the solidline and the broken line is larger. w=1.5 μm is the critical conditionat which the intensity variation occurs. Therefore, it is understoodthat the viewing angle dependence with which the light intensity in theplane-normal direction is strong, with which the variation in lightintensity relative to the declination (latitude) is moderate, and withwhich the light intensity difference in the longitude direction is smallcan be obtained under the condition that the boundary width w is 1.5 μmor less.

FIG. 16 illustrates the incidence angle dependence of the transmittancet of the surface structure 13 of the first embodiment. FIG. 16( a)illustrates how much of light whose light amount is 1 in the transparentsubstrate 5 and which impinges on the surface structure at the incidenceangle θ (the angle from the normal to the refracting surface) is emittedinto the air 6 in the first light extraction. FIG. 16( b) illustratesthe incidence angle dependence of the transmittance for the light whichis reflected by the surface structure 13 and then reflected by theelectrode 2 to impinge on the surface structure 13 again, i.e., theincidence angle dependence of the transmittance in the second lightextraction. In both diagrams, the refractive index of the transparentsubstrate 5 is n₁=1.457, the refractive index of the air 6 is n₀=1.0,the wavelength of light is λ=0.635 μm, the protrusion height of theminute regions δ₁ relative to the minute regions δ₂ is d=0.70 μm, andthe area proportion of the minute regions δ₁ (i.e., the proportion ofthe raised portions) is P=0.5. The width of the surface structure, w, isa parameter (w=0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, and 4.0 μm). Note thatthe protrusion height of d=0.70 μm is equivalent to a condition where,as for vertically incident light, the phase difference between lighttransmitted through the recessed portions and light transmitted throughthe raised portions is π (d=λ/2(n₁−n₀)).

The probabilities that each of the minute regions δ is the minute regionδ₁ or the minute region δ₂ are P and 1−P, respectively. Thus, among theminute regions δ, two or more minute regions δ₁ or minute regions δ₂successively adjoin one another. In this case, no boundary is formedbetween the adjoining minute regions δ₁ or minute regions δ₂, and theboundary is imaginary. However, the boundary between these regions isnot formed only because the minute regions δ₁ or minute regions δ₂ aresuccessively adjoining and, in this case also, it can be said that thesurface of the transparent substrate 5 is divided with one minute regionδ being the basic unit of the division.

The results shown in FIG. 16( a) are approximate to the results from the180°-phase shifter (FIG. 9), except for the results for w=0.1 μm and 0.2μm, so that the transmittance is large even if the critical angle isexceeded. FIG. 17 is an experimental result which illustrates theincidence angle dependence of the transmittance t when the incidentlight is P-polarization. The experiment was carried out with a quartzsubstrate in which a random pattern of recessed portions and raisedportions, depth d=0.70 μm and boundary width w=0.4 μm, was actuallyformed according to an electron beam method, with the use of themeasurement apparatus shown in FIG. 11. The experimental resultexhibited good agreement with the analysis result shown in FIG. 16( a),and it is understood that the transmittance is large even if thecritical angle (43.34°) is exceeded. As previously described in thesection provided before the present embodiment, if equivalent emissionof light (so called “boundary diffraction effect”) occurs on therefracting surface, the light would propagate through both media on theopposite sides of the refracting surface irrespective of the incidenceconditions on the refracting surface. The phenomenon illustrated in FIG.16, in which light is transmitted through the refracting surface even ifthe critical angle is exceeded, can be explained based on the conditionswhich allow occurrence of equivalent radiation of light on thisrefracting surface.

Assuming that point light radiation occurs and light uniformly diffusesinside the transparent substrate 5 in the form of a spherical wave, thetotal amount of light in the range of the directional angle of radiationfrom 0 (identical with the above-described incidence angle θ) to θ+dθ isproportional to sin θdθ. Therefore, the amount of extracted light isproportional to the transmittance t shown in FIGS. 16( a) and 16(b)multiplied by sin θ. FIGS. 18( a) and 18(b) illustrate the incidenceangle dependence of the amount of extracted light at the surfacestructure of the first embodiment. FIG. 18( a) illustrates how much oflight whose light amount is 1 and which is radiated from a point in thetransparent substrate 5 (in actuality, a point in the light emittinglayer) and impinges on the surface structure at the incidence angle θ(the angle from the normal to the refracting surface) is emitted intothe air 6 in the first light extraction. FIG. 18( b) illustrates theincidence angle dependence of the amount of extracted light for thelight which is once reflected by the surface structure 13 and thenreflected by the electrode 2 to impinge on the surface structure 13again, i.e., the incidence angle dependence of the amount of extractedlight in the second light extraction.

Here, the light extraction efficiency is obtained by integrating theamount of extracted light by the incidence angle θ. FIGS. 19( a) and19(b) illustrate the light extraction efficiency of the surfacestructure 13 of the first embodiment. The data are shown together, underthe same conditions as those of FIG. 16, with the abscissa axisrepresenting the boundary width w of the surface structure 13. FIG. 19(a) shows the light extraction efficiency for the protrusion height ofthe surface structure 13 d=0.70 μm, as well as for d=0.1, 0.30, 0.50,1.40 μm (the light extraction efficiency in the first light extraction,η₁), and the light extraction efficiency for light which is reflected bythe surface structure 13 and then reflected by the electrode 2 toimpinge on the surface structure 13 again on the assumption thatattenuation of light during its to-and-fro travel, such as absorption bythe transparent electrode 4 and reflection loss at the electrode 2,would not occur (the light extraction efficiency in the second lightextraction, η₂). The curves 5 a and 5A represent the light extractionefficiencies in the first and second light extractions for d=0.70 μm.The curves 5 b and 5B represent the light extraction efficiencies in thefirst and second light extractions for d=0.50 μm. The curves 5 c and 5Crepresent the light extraction efficiencies in the first and secondlight extractions for d=0.30 μm. The curves 5 g and 5G represent thelight extraction efficiencies in the first and second light extractionsfor d=0.10 μm. In this case, the light extraction efficiency is lowerthan those for the other depths, and hence, the protrusion height d needto be 0.20 μm or more. As illustrated by the curve 5 h, when d is equalto or greater than twice the visible light wavelength (d≧1.4 μm), theefficiency in the first light extraction greatly deteriorates in theregion where width w is 1.5 μm or less. Therefore, the protrusion heightd is preferably 1.4 μm or less. Thus, the recommended values of d arewithin the range of 0.2 μm to 1.4 μm. More generally speaking, thecondition of λ/(n₁−n₀)≧d≧λ/6(n₁−n₀) is the recommended value range forthe height difference, where n₁ is the refractive index of thetransparent substrate 5, n₀ is the refractive index of the air 6 (notethat the medium with which the transparent substrate 5 is in contactdoes not need to be the air, but the refractive index n₀ of the mediummay be smaller than the refractive index n₁ of the transparent substrate5), and λ is the center wavelength of the spectrum of light.

In the range of d≦0.70 μm, the light extraction efficiencies in thefirst light extraction achieve the local maximum when the boundary widthw is from 0.4 μm to 2 μm. As w is decreased or increased, the lightextraction efficiencies approach to 0.27 (which is a value of the lightextraction efficiency given by Formula 3 when the surface is a specularsurface). The light extraction efficiency in the second light extractionhas the local maximum value in the range from w=0.10 μm to w=2.0 μm andapproaches to 0.00 as w increases (although not shown in the range ofFIG. 19). In the range of w≦0.10 μm, it converges to 0.00 as wdecreases.

For comparison, in FIG. 19( b), the curves 5 d and 5D represent thelight extraction efficiencies in the first and second light extractionsunder the condition that the phase shifters which convert the phase oflight by 180° are provided in the minute regions δ₁ instead of thesurface structure 13. In the surface structure 13 of the presentembodiment, a phase difference occurs between the light transmittedthrough the recessed portions and the light transmitted through theraised portions during the transmission therethrough by the distanceequal to the height difference, whereas the phase shifter is capable ofproducing an equivalent phase difference with no transmission distance.In the case of the phase shifter, as the boundary width w increases, thelight extraction efficiencies in the first and second light extractionsapproach to 0.27 and 0.00, respectively, as they do for the surfacestructure 13. As w decreases to 0.3 μm or smaller, not only the lightextraction efficiency in the second light extraction but also the lightextraction efficiency in the first light extraction become zero (thereason for this has already been described with reference to FIG. 7(e)). One of the possible reasons for a higher light extractionefficiency obtained in the surface structure 13 of the presentembodiment, under the condition that the boundary width is 0.4 μm orless, than that achieved in the phase shifter is the function of theraised portions as a light guiding path.

The light extraction efficiency in the second light extraction withconsideration for attenuation of light during its to-and-fro travelbetween the surface of the transparent substrate 5 and the electrode 2is τ×η₂ where τ is the transmittance of light during its to-and-frotravel between the surface of the transparent substrate 5 and theelectrode 2 relative to the transparent substrate 5. Extraction of lightis not limited to once or twice but is repeated endlessly. On theassumption that the relationship is a geometric progression where thelight extraction efficiency in the first light extraction is η₁ and thelight extraction efficiency in the second light extraction is τ×η₂, thelight extraction efficiency for the n^(th) light extraction is expectedto be η₁×(τ×η₂/η₁)^(n-1). Thus, the total amount of extracted light upto the n^(th) light extraction is:

$\begin{matrix}{\eta_{1} \times {\sum\limits_{k = 1}^{n}\;\left( {\tau \times {\eta_{2}/\eta_{1}}} \right)^{k - 1}}} & \left( {{Formula}\mspace{14mu} 6} \right)\end{matrix}$When n is infinite, it approaches to η₁/(1−τ×η₂/η₁).

In FIG. 19( a), as for the curves 5 a and 5A (d=0.70 μm), when w=0.60μm, η₁=0.318 and η₂=0.093, and the light extraction efficiency obtainedfor τ=0.88 is 0.428. When w=1.00 μm, η₁=0.319 and η₂=0.102, and thelight extraction efficiency obtained is 0.444. On the other hand, in theconventional light emitting device shown in FIG. 1 and FIG. 3( a),η₁=0.274 and η₂=0, and for the second and subsequent incidences, theefficiencies are all zero; thus, the total light extraction efficiencyis 0.274. Therefore, it is understood that, under the condition ofw=0.60 μm, the light emitting device of the present embodiment achievesa light extraction efficiency that is 1.56 times that of the lightemitting device shown in FIG. 3( a). Under the condition of w=1.00 μm,the light emitting device of the present embodiment achieves a lightextraction efficiency that is 1.62 times that of the light emittingdevice shown in FIG. 3( a). Thus, when w is greater than 0.2 μm(generally, when the diameter of the largest inscribed circle of theminute regions δ is 0.2 μm or greater), the light extraction efficiencycan be greatly improved.

Next, the dependence on the wavelength of the light extractionefficiency of the surface structure 13 of the present embodiment isconsidered.

The curves 5 a′, 5A′, 5 h′, and 5H′ of FIG. 19( a) represent the lightextraction efficiencies in the first and second light extractions ford=0.70 μm and 1.40 μm under the condition where the wavelength is 0.45μm. These characteristics are substantially identical with the resultsfor the wavelength of 0.635 μm. Therefore, it is understood that thevariation in extraction efficiency due to the wavelength differencewithin the visible light range can be decreased.

Thus, the surface structure 13 of the present embodiment achieves alight extraction efficiency which is approximate to the optimum valuefor all the wavelengths within the visible light range even when thesurface structure 13 has a single shape (defined by d and w). Therefore,when this structure is used in the display surface of a display device,it is not necessary to prepare different shapes for respective ones ofthe three types of pixels of RGB, so that the configuration andadjustments made during assemblage can be greatly simplified.

In some organic EL elements, a transparent adjustment layer may beprovided on the transparent electrode 4 for adjusting the transmittanceof light during its to-and-fro travel between the transparent substrate5 and the electrode 2. In this case, the transparent substrate 5 isstacked on the adjustment layer (i.e., an organic EL element whichfurther includes the adjustment layer may be referred to as “lightemitting body”). When the refractive index n₁ of the transparentsubstrate 5 is smaller than the refractive index n₁′ of the adjustmentlayer, there is an interface between the transparent substrate 5 and theadjustment layer at which total reflection occurs. Especially whenn₁′−n₁>0.1, the interface produces a normegligible effect. FIG. 20 showsthe propagation of light under such a condition.

In FIG. 20, light radiated from the point S that is inside the lightemitting layer 3 of the refractive index n₂ enters the transparentelectrode 4 directly or after being reflected by the electrode 2 and isthen transmitted therethrough. Then, the light is transmitted through anadjustment layer 15 of the refractive index n₁′ and refracted at pointP′ on an interface 15 a. The refracted light is transmitted through thetransparent substrate 5 of the refractive index n₁ to be emitted intothe air 6 via point P on the interface between the transparent substrate5 and the air 6. Here, n₁′≧n₂>n₁>1.0. Note that n₁′ may be smaller thann₂, but in such a case, total reflection occurs between the transparentelectrode 4 and the adjustment layer 15. Since a surface of thetransparent substrate 5 bordering on the air 6 has the surface structure13 of the present embodiment, light that is incident at an angle greaterthan the critical angle can also be extracted to the air layer 6 side.However, due to the relationship of n₁′>n₁, total reflection also occurson the interface 15 a. Specifically, incidence of light at point Q′which occurs with a greater incidence angle than at point P′ results intotal reflection. This reflected light repeatedly undergoes totalreflection between the interface 15 a and the electrode 2 and thereforecannot be extracted to the air 6 side.

In such a case, as shown in FIG. 21, another surface structure 13′ ofthe present embodiment is provided at the interface between theadjustment layer 15 and the transparent substrate 5, whereby lightimpinging on this surface at an incidence angle greater than thecritical angle can be extracted to the air 6 side. Specifically, due tothe surface structure 13′, incidence of light at point Q′ at an anglegreater than the critical angle does not result in occurrence of totalreflection. A component of the light which is reflected by this surfaceis then reflected by the electrode 2 to again impinge on the surfacestructure 13′ at point R′. Part of the impinging light can be emittedinto the air 6 via the surface structure 13. This process is repeatedendlessly. The configuration of FIG. 21 has complexity in forming twolayers of the surface structures 13, 13′ which have recessed portionsand raised portions. However, the configuration of FIG. 21 isadvantageous because a material of a low refractive index can be usedfor the transparent substrate 5, so that a wider selection of materialscan be provided.

Note that, as seen from formula 6, as the transmittance τ of lightduring its to-and-fro travel between the transparent substrate 5 and theelectrode 2 increases, the light extraction efficiency increases. In anactual device, the light emitting layer 3 is surrounded by a pluralityof transparent layers, such as the above-described adjustment layer 15,as well as by the electrode 2 and the transparent electrode 4. Thedesign of these films (the refractive index and the thickness of thefilms including the light emitting layer 3) is to be determined suchthat the above-described transmittance τ achieves the maximum value. Inthis case, reflection by the surface structure 13 results in a randomphase distribution, and therefore, superposition of reflected lightbeams is considered as being incoherent (it is not addition ofamplitudes but addition of intensities). Thus, the effects of thereflection by the surface of the transparent substrate 5 are negligible,and therefore, it can be assumed that the reflectance is 0% and thetransmittance is 100%. The refractive index and the thickness ofrespective one of the films are determined so as to maximize the amountof light obtained by superimposition of complex amplitudes of lightbeams which are radiated from the transparent substrate 5 and repeatedlytravel to-and-fro through a multilayer film including the light emittinglayer 3 to return to the transparent substrate 5 under the aboveassumption.

Second Embodiment

The second embodiment is described with reference to FIG. 22 and FIG.23. Note that the second embodiment is different from the firstembodiment only in the pattern of the surface structure 13, and theother elements are all the same as those of the first embodiment. Thedescriptions of the common elements are herein omitted.

In the second embodiment, the proportion of the raised portions of thesurface structure, P, and the proportion of the recessed portions, 1−P,are not fixed to 0.5 but are within the range of P=0.4 to 0.98.Specifically, the minute regions δ₁ (the regions protruding above thesurface) constitute 40% to 98%, while the minute regions δ₂ (recesses)constitute 60% to 2%.

FIG. 22( a) illustrates the light extraction efficiency of the surfacestructure of the present embodiment. The refractive index of thetransparent substrate 5 is n₁=1.457. The refractive index of the air 6is n₀=1.0. The wavelength of light is λ=0.635 μm. The protrusion heightof the surface structure is d=0.70 μm. The abscissa axis represents theboundary width w of the surface structure. The light extractionefficiencies for the proportion P=0.2, 0.4, 0.6, 0.8, 0.9 (in the firstand second light extractions). The curves 6 a, 6 b, 6 c, 6 d, 6 e and6A, 6B, 6C, 6D, 6E represent the light extraction efficiencies forP=0.2, 0.4, 0.6, 0.8, 0.9, respectively. The curves 27 a and 27A of FIG.23 represent the light extraction efficiencies (in the first and secondlight extractions) which are plotted over the abscissa axis thatrepresents the proportion of the raised portions, P, under the aboveconditions for the boundary width of w=1.0 μm.

As seen from FIG. 22( a), in terms of the light extraction efficiency inthe first light extraction, the characteristic for the proportion P=0.2is the lowest in the entire range of w. In the range of w≦2 μm, thecharacteristic for P=0.6 exhibits the largest value. In terms of thelight extraction efficiency in the second light extraction, in the rangeof w≦4 μm, the characteristic for P=0.9 is the highest, and thecharacteristic for P=0.2 is the lowest.

As seen from the curve 27 a of FIG. 23, in the first light extraction,the light extraction efficiency can be further improved by setting theproportion P, which determines the area ratio between the recessedportions and the raised portions, within a range centered at 0.6 andranging from 0.4 to 0.8. This is presumably because the raised portionseffectively function as a light guiding path in this range. (When P≦0.2,the area proportion of the raised portions which constitute a lightguiding path is small. When P≧0.8, adjacent raised portions are tooclose to one another so that the wave guiding effect decreases.) On theother hand, as seen from the curve 27A of FIG. 23, as for the secondlight extraction, the light extraction efficiency can be furtherimproved by setting the proportion P within a range centered at 0.9 andranging from 0.5 to 0.98. Thus, in terms of the total light extractionefficiency including the light extraction efficiencies in the first andsecond light extractions, the proportion P is preferably set within therange of 0.4 to 0.98.

Thus, in the present embodiment, the proportion P is deviated from 0.5,whereby a higher light extraction efficiency than that of the firstembodiment can be obtained. Also, as in the first embodiment, variationin light intensity distribution and color imbalance do not occurdepending on the direction, and the light extraction efficiency can begreatly improved. Also, reflection of an ambient image can be prevented.

Third Embodiment

The third embodiment is described with reference to FIG. 22( b). Thethird embodiment is different from the first and second embodiments onlyin the height difference conditions of the surface structure 13, and theother elements are all the same as those of the first and secondembodiments. The descriptions of the common elements are herein omitted.

In the third embodiment, the difference in height between two adjacentminute regions δ₁, δ₂ of the surface structure of the first or secondembodiment varies randomly. The method of making such a randomarrangement is realized by dividing the surface of the transparentsubstrate 5 into boxes of a chessboard-like grid (square minute regionsδ) without leaving any gap therebetween as shown in FIG. 13( a), eachbox having the width w (herein referred to as “boundary width”), andrandomly allocating arbitrary heights (or depths) within a range from−d_(m)/2 to d_(m)/2 relative to a single reference plane respective onesof the boxes according to a random function. The single reference planemay be a plane which is parallel to the surface of the transparentsubstrate 5 and which is equidistant from a highest one of the minuteregions δ and a lowest one of the minute regions δ in terms of adirection parallel to the normal to the surface of the transparentsubstrate 5. d_(m) represents the difference in position in terms of aheight direction between the highest one of the minute regions δ and thelowest one of the minute regions δ.

FIG. 22( b) illustrates the light extraction efficiency of the surfacestructure of the present embodiment. The refractive index of thetransparent substrate 5 is n₁=1.457. The refractive index of the air 6is n₀=1.0. The wavelength of light is λ=0.635 μm. The light extractionefficiencies η₁ in the first light extraction and the light extractionefficiencies η₂ in the second light extraction, for the maximum heightdifferences d_(m)=1.4, 0.9, 0.7, and 0.3 μm, are shown over the abscissaaxis that represents the boundary width of the surface structure (thewidth of the minute regions δ) w. For the convenience of calculation,the randomness in height difference relative to the reference plane isdetermined such that four height difference values are randomly selected(the probability of occurrence of each height difference value is 25%)as follows: for d_(m)=1.4 μm, 4 different height difference values from−0.7 μm to 0.7 μm at intervals of 0.467 μm; for d_(m)=0.9 μm, differentheight difference values from −0.45 μm to 0.45 μm at intervals of 0.3μm; for d_(m)=0.7 μm, 4 different height difference values from −0.35 μmto 0.35 μm at intervals of 0.233 μm; and for d_(m)=0.3 μm, 4 differentheight difference values from −0.15 μm to 0.15 μm at intervals of 0.1μm. Note that the probabilities of occurrence of the respective steps donot need to be equal. For example, the probabilities of occurrence ofsteps at lower (deeper) positions may be greater, while theprobabilities of occurrence of steps at higher (shallower) positions maybe smaller.

The curves 6 i and 6I represent the light extraction efficiencies in thefirst and second light extractions, respectively, for d_(m)=1.4 μm. Thecurves 6 h and 6H represent the light extraction efficiencies in thefirst and second light extractions, respectively, for d_(m)=0.9 μm. Thecurves 6 g and 6G represent the light extraction efficiencies in thefirst and second light extractions, respectively, for d_(m)=0.7 μm. Thecurves 6 f and 6F represent the light extraction efficiencies in thefirst and second light extractions, respectively, for d_(m)=0.3 μm. Asin the first example, the light extraction efficiency in the first lightextraction achieves the local maximum when the boundary width w is from0.2 μm to 2 μm. As w is decreased or increased, the extractionefficiency approaches to 0.27 (which is a value of the light extractionefficiency given by Formula 3 when the surface is a specular surface).The light extraction efficiency in the second light extraction convergesto 0.00 as w decreases in the range of w≦0.20 μm. The light extractionefficiency approaches to 0.00 as w is increased from 8 μm, although notshown. Therefore, the range of the boundary width w needs to be 0.2 μmor greater. Further, as discussed with reference to FIG. 14 and FIG. 15in conjunction with the first example, it is preferably 1.5 μm or lessin consideration of the viewing angle dependence. In FIG. 22( b), thelight extraction efficiencies in the first and second light extractions(η₁, η₂) calculated under the conditions of d_(m)=0.7 μm and theboundary width w=0.6 μm are 0.331 and 0.141, respectively. Therefore,the characteristic obtained for d_(m)=0.7 μm is better in terms of thelight extraction efficiency in the second light extraction than thecharacteristic obtained in the first embodiment (curve 5A) and thecharacteristic obtained in the second embodiment (curves 6B and 6C).This is presumably because the tip ends of the raised portions are atdifferent heights so that the randomness of the pattern increases.Accordingly, the randomness of the propagation direction of lightreflected by the surface structure increases so that the diffusion ofthe reflected light improves. As a result, even in the second lightextraction, light can be incident on the surface structure inapproximately the same conditions as those of the first light extraction(the light intensity is uniform among all the directions).

In the range of w≧0.4 μm, the characteristic for d_(m)=0.30 μm in thefirst light extraction is inferior to that for d_(m)=0.70 μm. Therefore,d_(m) is preferably d_(m≧)0.2 μm to 0.3 μm. (This range is equal to thatof the first embodiment). In the range of w≦1.0 μm, the characteristicfor d_(m)=1.40 μm in the first light extraction is slightly better thanthat for d_(m)=0.70 μm. However, when d_(m) is too large, the processingis difficult, and the viewing angle characteristic deteriorates in thecondition of w≦1.5 μm (see FIG. 14 and FIG. 15). Thus, 1.40 μm may be anappropriate value for the upper limit of d_(m). These ranges are equalto the range of the first embodiment (λ/(n₁−n₀)≧d_(m)≧λ/6(n₁−n₀)).

As described hereinabove, in the third embodiment, the height differenceis randomly varying, so that a higher light extraction efficiency thanthe first and second embodiments can be obtained. Also, as in the firstembodiment, variation in light intensity distribution and colorimbalance do not occur depending on the direction. Also, reflection ofan ambient image can be prevented.

There are two possible conditions for the random height difference: (1)the height difference can have all the values from 0 to the maximumheight difference d_(m); and (2) the height difference can have any ofthree or more height difference values including 0 and the maximumheight difference d_(m). As an example of the condition (2), considerthat the four different height difference values 0, d_(m)/3, d_(m)×2/3,and d_(m) are possible. A mold for shape transfer which is configured toform such a surface structure over a sheet surface can be fabricated bytwo cycles of an exposure-etching step (1st cycle: exposure with maskpattern of boundary width w₁ and etching to the depth of d_(m)/3; 2ndcycle: exposure with mask pattern of boundary width w₂ and etching tothe depth of d_(m)×2/3). Here, w₂=w₁ is the condition for maximizing thefrequency of occurrence of discontinued boundary lines.

Now, consider a case where the seven different heights (heightdifferences), 0, d_(m)/6, d_(m)×2/6, d_(m)×3/6, d_(m)×4/6, d_(m)×5/6,d_(m), are possible. A mold for shape transfer which is configured toform such a surface structure over a sheet surface can be fabricated bythree cycles of an exposure-etching step (1st cycle: exposure with maskpattern of boundary width w₁ and etching to the depth of d_(m)/6; 2ndcycle: exposure with mask pattern of boundary width w₂ and etching tothe depth of d_(m)×2/6; 3rd cycle: exposure with mask pattern ofboundary width w₃ and etching to the depth of d_(m)×3/6). Here, w₁=w₂=w₃is the condition for maximizing the frequency of occurrence ofdiscontinued boundary lines.

Fourth Embodiment

The fourth embodiment is described with reference to FIG. 24. Note thatthe fourth embodiment is different from the first embodiment only in thepattern of the surface structure, and the other elements are all thesame as those of the first embodiment. The descriptions of the commonelements are herein omitted.

FIG. 24 illustrates the process of determining the pattern of thesurface structure in the fourth embodiment. FIG. 24( a) shows thesurface of the transparent substrate 5 which is divided into boxes of achessboard-like grid (square minute regions α), each box having widthw₁. To respective ones of the boxes, white and black are randomlyallocated such that each of the proportion of black boxes and theproportion of white boxes is 50%. In the shown example, w₁=1 μm (notethat the description below is given with this value for the sake ofvisibility of the drawing, although the optimum value for w₁ is farsmaller than this). Some of the minute regions α to which black isallocated are referred to as “minute regions α₁”, and the other minuteregions α to which white is allocated are referred to as “minute regionsα₂”.

FIG. 24( b) shows the surface of the transparent substrate 5 which isdivided into boxes of a chessboard-like grid (square minute regions β),each box having width w₂ that is an integral multiple of w₁. Torespective ones of the boxes, white and black are randomly allocatedsuch that the proportion of black boxes is P2, the proportion of whiteboxes is 1−P₂, and P₂=0.5. In the shown example, w₂=2 μm. Some of theminute regions β to which black is allocated are referred to as “minuteregions β₁”, and the other minute regions β to which white is allocatedare referred to as “minute regions β₂”.

FIG. 24( c) shows a pattern generated by superimposing the patterns ofFIG. 24( a) and FIG. 24( b) such that the boxes are in alignment betweenthese patterns according to a rule that superimposition of a blackregion (α₁) and a black region (β₁) is to be replaced by a white box,superimposition of a white region (α₂) and a white region (β₂) is to bereplaced by a white box, and superimposition of a white region (α₂) anda black region (β₁) and superimposition of a black region (α₁) and awhite region (β₂) are to be replaced by black boxes. As a result, thepattern generation rule of FIG. 24( c) is the same as that of thepattern of FIG. 24( a). The pattern of the surface structure in whichthe black boxes correspond to raised portions and, relatively, the whiteboxes correspond to recessed portions is the same as that described inthe first embodiment.

FIG. 24( d) shows the surface of the transparent substrate 5 which isdivided into boxes of a chessboard-like grid (square minute regions α),each box having width w₁. To respective ones of the boxes, white andblack are randomly allocated such that the proportion of black boxes isP₁ and the proportion of white boxes is 1−P₁. In the shown example, w₁=1μm and P₁=0.1. As in FIG. 24( a), the black boxes are referred to as“minute regions α₁”, and the white boxes are referred to as “minuteregions α₂”.

FIG. 24( e) shows a pattern generated by superimposing the patterns ofFIG. 24( d) and FIG. 24( b) such that the boxes are in alignment betweenthese patterns according to a rule that superimposition of a blackregion (α₁) and a black region (β₁) is to be replaced by a white box,superimposition of a white region (α₂) and a white region (β₂) is to bereplaced by a white box, and superimposition of a white region (α₂) anda black region (β₁) and superimposition of a black region (α₁) and awhite region (β₂) are to be replaced by black boxes. The pattern of FIG.24( e) has similar features to those of the pattern of FIG. 24( c). Forexample, the area ratio between black and white is 1:1, and the minimumsizes of the black marks and the white marks are equal. However, thepattern of FIG. 24( e) is different from the pattern of FIG. 24( c) inthat the proportion of the minimum size marks is smaller. The finallydetermined black-white ratio (the area ratio between recessed portionsand raised portions) depends on the proportions P₁ and P₂. Theproportion of the black marks (the proportion of the raised portions),P, is given as P=P₁+P₂−2P₁P₂.

In FIG. 23, the curves 27 b and 27B represent the characteristics of thelight extraction efficiencies calculated for the first and second lightextractions (η₁, η₂), which are plotted over the abscissa axis thatrepresents the proportion of the raised portions, P, under theconditions that the protrusion height of the raised portions of thesurface structure is d=0.70 μm, w₁=0.2 μm, w₂=1 μm, and P₁=0.1.

As seen from the curve 27 b of FIG. 23, in the first light extraction,the light extraction efficiency can be further improved by setting theproportion P, which determines the area ratio between the recessedportions and the raised portions, within a range centered at 0.6 andranging from 0.4 to 0.8, notwithstanding the distribution of therecessed and raised portions has a different pattern from that of thefirst example. On the other hand, as seen from the curve 27B, in thesecond light extraction, the light extraction efficiency can be furtherimproved by setting the proportion P within a range from 0.5 to 0.9(note that the curves 27 b and 27B cannot be plotted in the range of 0.1or less and in the range of 0.9 or more because P₁=0.1). Thus, the totallight extraction efficiency, including the efficiencies in the first andsecond light extractions, can be improved by setting the proportion P,which is finally obtained from a combination of the proportions P₁ andP₂, within the range of 0.5 to 0.98 as in the first example. In FIG. 23,the curves 27 c and 27C represent the characteristics of the lightextraction efficiencies in the first and second light extractions,respectively, under the conditions that w₁=0.1 μm and P₁=0.1. The curves27 d and 27D represent the characteristics of the light extractionefficiencies in the first and second light extractions, respectively,under the conditions that w₁=0.1 μm and P₁=0.2. Since the efficienciesgreatly deteriorate when w₁ is less than 0.2 μm, w₁ needs to be 0.2 μmor more. The upper limit for w₁ is preferably 1.5 μm or less inconsideration of the viewing angle dependence as discussed inconjunction with the first example with reference to FIG. 14 and FIG.15.

The fourth embodiment is different from the first embodiment in thatsmall changes are made in the formation conditions of the surfacestructure. The light extraction efficiency may be slightly inferior tothat of the first embodiment under some conditions. However, the fourthembodiment can still achieve a light extraction efficiency which is muchhigher than that of the conventional light emitting device shown in FIG.1 and FIG. 3( a). As in the first embodiment, the fourth embodiment hasthe following advantages: variation in light intensity distribution andcolor imbalance do not occur depending on the direction; the lightextraction efficiency can be greatly improved; and reflection of anambient image can be prevented. The fourth embodiment has moderaterequirements for the shape of the surface structure as compared with thefirst embodiment, so that a large error margin can be secured, whichwill be a merit in terms of easy processing. For example, under theconditions of the first embodiment, the interval between adjacentrecessed portions or the interval between adjacent raised portions isshorter, and therefore, the processing of a fine shape of the recessedor raised portions is relatively difficult. However, in the fourthembodiment, the proportion of the fine recessed or raised portions issmall (see FIGS. 24( c) and 24(e)). Therefore, the interval betweenadjacent recessed portions or the interval between adjacent raisedportions is effectively larger, so that a hurdle for the difficulty inprocessing is lowered. As a matter of course, application of the fourthembodiment to the second embodiment would achieve the same effects asthose of the second embodiment.

Fifth Embodiment

The fifth embodiment is a combination of the fourth embodiment and thethird embodiment. In the present embodiment, for the sake of easyrecognition of allocation of regions, the respective regions aredistinguished by color in the description below. In the fifthembodiment, firstly, the surface of the transparent substrate 5 isdivided into boxes of a chessboard-like grid (square minute regions α)such that each box has the width w, and black and white are randomlyallocated to respective ones of the boxes such that the proportion ofblack boxes is P₁ and the proportion of white boxes is 1−P₁. Regions towhich white is allocated (minute regions α₂) are lowered by the depth ofd₁ (>0) by means of, for example, etching. Regions to which black isallocated are referred to as “minute regions α₁”.

Then, the surface of the transparent substrate 5 is divided into boxesof a chessboard-like grid (square minute regions β) such that each boxhas the width w₂, and blue and red are randomly allocated to respectiveones of the boxes such that the proportion of blue boxes is P₂ and theproportion of red boxes is 1−P₂. Regions to which red is allocated(minute regions (β₂) are lowered by the depth of d₂ (>0) by means of,for example, etching. Regions to which blue is allocated are referred toas “minute regions β₁”. Note that the width w₂ is an integral multipleof the width w₁ (w₂=w₁ is the most preferred case). The twochessboard-like grids are superimposed with each other such that theboundary lines are in alignment between the grids.

By superimposition of the grids, relative to the surface of an area inwhich a white region and a red region are superimposed with each other(herein referred to as “reference plane”), superimposition of a blackregion and a blue region has a height of d₁+d₂, superimposition of awhite region and a blue region has a height of d₂, and superimpositionof a black region and a red region has a height of d₁. Therefore, theheight differences can randomly have the four values from 0 to d₁+d₂ (0,d₁, d₂, d₁+d₂), so that the same effect as those of the third embodimentcan be achieved.

Under the conditions of d₁=d_(m)×1/3 and d₂=d_(m)×2/3, the pattern ofthe width w₁, which has a finer structure and therefore has difficultyin processing, can have a decreased depth, which will be a merit interms of easy processing. Under the conditions of d₁=d_(m)×1/3 andd₂=d_(m)×2/3, the proportion P₂ has a similar meaning to that of theproportion P₂ of the fourth embodiment which determines the area ratiobetween the recessed portions and the raised portions, i.e., the averagelevel of the depth because it corresponds to the more deeply loweredside (in actuality, the proportion P₂ side relates to the average depthwith a weight of 2, while the proportion P₁ side relates to the averagedepth with a weight of 1). On the other hand, the proportion P₁ relatesto the proportion of the finer structures (width w₁) and therefore has asimilar meaning to that of the proportion P₁ of the fourth embodiment.

In the above example, 2 sets of conditions of the exposure-etching stepare combined. When 3 sets of conditions of the exposure-etching step arecombined, the height can have a value randomly selected from 8 differentvalues. In this case, the following steps are added to theabove-described two etching steps. Specifically, the surface of thetransparent substrate 5 is divided into boxes of a chessboard-like grid(square minute regions γ) such that each box has the width w₃, andyellow and green are randomly allocated to respective ones of the boxessuch that the proportion of green boxes is P₃ and the proportion ofyellow boxes is 1−P₃. Regions to which yellow is allocated (minuteregions γ₂) are lowered by the depth of d₃ (>0) by means of, forexample, etching. Regions to which green is allocated are referred to as“minute regions γ₁”. Note that the width w₃ is an integral multiple ofthe width w₂ (w₃=w₂ is the most preferred case). The two chessboard-likegrids are superimposed with each other such that the boundary lines arein alignment between the grids.

By superimposition of the grids, relative to the surface of an area inwhich a white region, a red region, and a yellow region are superimposedwith one another (herein referred to as “reference plane”),superimposition of a black region, a blue region, and a green region hasa height of d₁+d₂+d₃, superimposition of a white region, a blue region,and a green region has a height of d₂+d₃, superimposition of a blackregion, a blue region, and a yellow region has a height of d₁+d₂,superimposition of a black region, a red region, and a green region hasa height of d₁+d₃, superimposition of a black region, a red region, anda yellow region has a height of d₁, superimposition of a white region, ablue region, and a yellow region has a height of d₂, and superimpositionof a white region, a red region, and a green region has a height of d₃.Therefore, the height differences can randomly have the eight valuesfrom 0 to d₁+d₂+d₃ (0, d₁, d₂, d₃, d₁+d₂, d₂+d₃, d₃+d₁, d₁+d₂+d₃), sothat the same effect as those of the third embodiment can be achieved.

Under the conditions of d₁=d_(m)×1/6, d₂=d_(m)×2/6, and d₃=d_(m)×3/6,the patterns of the widths w₁ and w₂, which have finer structures andtherefore have difficulty in processing, can have decreased depths,which will be a merit in terms of easy processing. Under the conditionsof d₁=d_(m)×1/6, d₂=d_(m)×2/6, and d₃=d_(m)×3/6, P₂ and P₃ have similarmeanings to that of the proportion P₂ of the fourth embodiment whichdetermines the area ratio between the recessed portions and the raisedportions, i.e., the average level of the depth because the proportionsP₂ and P₃ correspond to the more deeply lowered sides (in actuality, theproportion P₃ side relates to the average depth with a weight of 3, theproportion P₂ side relates to the average depth with a weight of 2, andthe proportion P₁ side relates to the average depth with a weight of 1).On the other hand, the proportion P₁ relates to the proportion of thefiner structures (width w₁) and therefore has a similar meaning to thatof the proportion P₁ of the fourth embodiment.

Sixth Embodiment

The sixth embodiment is described with reference to FIG. 12. Note thatthe sixth embodiment is different from the first embodiment only in thepattern of the surface structure 13, and the other elements are all thesame as those of the first embodiment. The descriptions of the commonelements are herein omitted.

In the sixth embodiment, two adjacent minute regions δ₁, δ₂ in thesurface structure of the first embodiment are formed by phase shifters.The phase shifters may be formed by, for example, a multilayer filmwhich includes layers of different refractive indices. Specifically, thephase of light transmitted through the multilayer film is adjusted bymeans of multiple reflection caused by the multilayer film. By changingthe configuration of the multilayer film (the film thickness and thenumber of layers), regions of 180° and regions of 0° can be randomlyformed. Also, the same effects can be obtained by using polarizers tochange the polarization of light transmitted through the two types ofregions. Here, the polarizers used herein are configured such that thepolarization of transmitted light corresponding to the 180° regions isP-polarization or right-handed circular polarization, and thepolarization of transmitted light corresponding to the 0° regions isS-polarization or left-handed circular polarization. These polarizerscan be realized by ½-wave plates whose directions are different by 90°.When using the uneven structure of recessed portions and raised portionswhich are formed at the interface between media of different refractiveindices as in the first example, the phase of transmitted light isvarying between the recessed portions and the raised portions, and thus,such an uneven structure can also be classified as one form of the phaseshifter.

The incidence angle dependence of the transmittance t and the lightextraction efficiency of the surface structure 13 in the presentembodiment have already been illustrated in FIG. 9 and FIG. 19( b)(curves 5 d and 5D). The light extraction efficiency in the first lightextraction can exceed the light extraction efficiency which would beachieved when w is in the range of 0.4 μm to 1 μm and the surface is aspecular surface. FIG. 19( b) also shows the results obtained when thephase difference is 90°. The light extraction efficiencies in the firstand second light extractions are represented by the curves 5 d′ and 5D′.The both light extraction efficiencies are inferior to those for thephase difference of 180° (curves 5 d and 5D). Thus, it is understoodthat the optimum value for the phase difference is 180°.

In this way, the surface structure 13 of the sixth embodiment is formedby phase shifters, whereby a higher light extraction efficiency thanthat of the conventional example can be achieved. Also, as in the firstembodiment, variation in light intensity distribution and colorimbalance do not occur depending on the direction, and reflection of anambient image can be prevented.

Seventh Embodiment

The seventh embodiment is described with reference to FIG. 25. Note thatthe seventh embodiment is different from the first embodiment only inthe pattern of the surface structure, and the other elements are all thesame as those of the first embodiment. The descriptions of the commonelements are herein omitted.

FIG. 25( a) shows a pattern diagram of a first surface structure 23according to the present embodiment. As shown in FIG. 25( a), thesurface of the transparent substrate 5 is patterned by a plurality oflinear segments (extending in the y′ direction) which are arranged sideby side along the x′ direction (that is perpendicular to the y′direction) across the surface without leaving any gap therebetween, eachof the linear segments being formed by minute regions δ which are in aone-dimensionally random arrangement. The surface structure 23 of thepresent embodiment is obtained by dividing the surface of thetransparent substrate 5 into right triangles (minute regions δ) suchthat each side of the triangle has a length w, and randomly allocatingraised portions (dotted boxes 23 a in the drawing (minute regions δ₁))or recessed portions (white boxes 23 b in the drawing (minute regionsδ₂)) to respective ones of the minute regions δ such that the proportionof the raised portions or recessed portions is 50%. Here, w is 2.55 μmor less. Generally speaking, the requirement for the size of the minuteregions is that the largest one of the inscribed circles of the minuteregions has a diameter of from 0.2 μm to 1.5 μm.

In FIGS. 25( a) and 25(b), a plurality of minute regions δ adjoining oneanother along the direction y′ are at positions which are coincidentwith one another in terms of the direction x′. Meanwhile, a plurality ofminute regions δ adjoining one another along the direction x′ are atpositions which are coincident with one another in terms of thedirection y′. In FIG. 25( c), a plurality of minute regions δ adjoiningone another along the direction y′ are at positions which are coincidentwith one another in terms of the direction x′. Meanwhile, a plurality ofminute regions δ adjoining one another along the direction x′ are atpositions which are different from one another in terms of the directiony′.

The seventh embodiment is different from the first embodiment only inthe shape of the pattern of the surface structure 23. The seventhembodiment is however based on the same principles as those of the firstembodiment and produces the same effects as those of the firstembodiment. The shape of the minute regions is not limited to a righttriangle but may be any polygon so long as the surface can be dividedinto geometrically congruent polygonal regions without leaving any gaptherebetween.

Note that, in actually manufactured products according to the first toseventh embodiments, the surface structures 13, 23 may not strictly havea right square or right triangular shape. For example, a vertex of onepolygonal minute region may be rounded, and a vertex of a minute regionadjacent to the vertex-rounded region may be accordingly deformed.However, even in such a case, as a matter of course, the characteristicsdo not degrade, and the same effects can be achieved. The variations ofthe second to sixth embodiments to which the seventh embodiment isapplied can achieve the same effects as those of the second to sixthembodiments.

Eighth Embodiment

The eighth embodiment is described with reference to FIG. 26 to FIG. 29.The eighth embodiment is a method of forming a random pattern shape of asheet.

(Description of Cylindrical Mold)

FIG. 26( a) shows a cylindrical mold for use in the present embodiment.The cylindrical mold 31 shown in FIG. 26( a) is used in a so-calledroll-to-roll transfer. The cylindrical mold 31 is rotatable around acenter axis 30. The side surface of the cylindrical mold 31 has recessedportions and raised portions (white regions 31 a, black regions 31 b)for formation of the minute regions δ. The cylindrical mold 31 isrotated while a sheet 31 c made of, for example, a resin (shown in FIGS.26( b) and 26(c)) is pressed against the side surface of the cylindricalmold 31, whereby the surface structure of the cylindrical mold 31 istransferred to the resin sheet.

FIGS. 27( a) to 27(d) are enlarged views of part of the side surface ofthe cylindrical mold 31. FIG. 27( a) shows the one-dimensionalarrangement of minute regions δ formed in the side surface of thecylindrical mold 31 (tape-like linear segment). As shown in FIG. 27( a),the minute regions δ are one-dimensionally arranged over the cylindricalmold 31, and both the width and the length of the minute regions δ arew. The minute regions δ are classified into minute regions δ₁ (dottedregions 33 a in the drawing) and minute regions δ₂ (white regions 33 bin the drawing). The minute regions δ₁ and the minute regions δ₂ are ina one-dimensionally random arrangement. The dotted regions 33 arepresent raised portions formed in the side surface of the cylindricalmold 31. The white regions 33 b represent recessed portions formed inthe side surface of the cylindrical mold 31. The recessed portions andthe raised portions may be inverted.

FIG. 28( a) illustrates the direction of the arrangement of the minuteregions δ formed in the side surface of the cylindrical mold 31. Asshown in FIG. 28( a), the linear segment transfer pattern of the minuteregions δ (shown in FIG. 27( a)) are spirally provided over the sidesurface of the cylindrical mold 31. In other words, the linear segmenttransfer pattern is formed continuously, in a single-stroke fashion,over the side surface of the cylindrical mold 31. The direction of thearrangement of the transfer pattern is along the direction y′ that isinclined by a small angle θ from the direction y perpendicular to therotation axis 30. Since the direction of the arrangement of the transferpattern is inclined by a small angle θ, the linear segment transferpattern can be arranged so as to form a single spiral line.

As previously described, the minute regions δ have a planar shape of apolygon such as a triangle or rectangle. When the planar shape of theminute regions δ is a right square or rectangle, two out of the foursides of the minute region δ extend along the direction y′ that isinclined by a small angle θ from the direction y perpendicular to therotation axis 30, while the other two sides extend along the directionx′ that is inclined by a small angle θ from the direction x parallel tothe rotation axis 30 (i.e., along the direction that is perpendicular tothe direction y′). On the other hand, when the planar shape of theminute regions δ is a polygon such as a triangle, the minute regions δare arranged such that one side of the minute region δ extends along thedirection y′. In these cases, plural ones of the minute regions δadjoining one another along the direction y′ are at positions which arecoincident with one another in terms of the direction x′. On the otherhand, plural ones of the minute regions δ adjoining one another alongthe direction x′ may be at positions which are coincident with oneanother in terms of the direction y′ (FIGS. 25( a) and 25(b) and FIG.27( b)) or may be at positions which are different from one another interms of the direction y′ (FIG. 25( c) and FIGS. 27( c) and 27(d)).

Note that, even when a gap is left between the minute regions δ formedin the side surface of the cylindrical mold 31 or an overlap of theminute regions δ occurs due to the manufacturing accuracy, a detrimentalproblem does not occur in manufacture of a sheet because a randompattern is generally formed over the entire sheet.

FIG. 28( b) shows a sheet 32 in which the uneven shape of recessed andraised portions formed in the side surface of the cylindrical mold 31 asshown in FIG. 28( a) is transferred. As shown in FIG. 28( b), over thesurface of the sheet 32, the minute regions δ are randomly arrangedalong the direction y′ that is inclined by a small angle θ from thedirection y parallel to the longer sides of the sheet 32. When theplanar shape of the minute regions δ is right square or rectangular, twoout of the four sides of the minute regions δ extend along the directiony′, while the remaining two sides extend along the direction x′ that isinclined by a small angle θ from the direction x parallel to the shortersides of the sheet 32. On the other hand, the planar shape of the minuteregions δ is polygonal, such as triangular or the like, the minuteregions δ are arranged such that one side of the minute region δ extendsalong the direction y′. Note that the minute regions δ are formed alongthe direction as shown in FIG. 28( b) only when the cylindrical mold 31is rotated while the longer sides of the sheet 32 are coincident with adirection perpendicular to the rotation axis of the cylindrical mold 31.If the sheet 32 were arranged otherwise, the arrangement of the minuteregions δ over the sheet 32 would be along a different direction. Thedescriptions of the features of the sheet 32 of the present embodimentwhich are the same as those of the sheets of the first to seventhembodiments are omitted herein.

In the side surface of the cylindrical mold 31 shown in FIG. 28( a) andthe sheet 32 shown in FIG. 28( b), the pattern is not entirely shown butonly in part of the surface. As a matter of course, the pattern ispreferably formed across the entire side surface of the cylindrical mold31. When the pattern is formed across the entire side surface of thecylindrical mold 31, the transfer pattern is formed in a region of thesheet 32 corresponding to the width of the side surface of thecylindrical mold 31. For example, when the width of the cylindrical mold31 and the width of the sheet 32 are equal, the transfer pattern can beformed across the entire sheet 32.

The manufacturing method of the present embodiment enables convenientformation of a sheet which has a large area and a pattern of highaccuracy. In the sheet 32 which is formed according to the manufacturingmethod of the present embodiment, the viewing angle dependence of lightextracted from the sheet 32 is small, and therefore, the in-planedirection dependence of the light extraction efficiency can bedecreased.

FIGS. 26( b) and 26(c) schematically illustrate the methods oftransferring the recessed and raised portions formed in the surface ofthe cylindrical mold 31 to a sheet 31 c based on a roll-to-roll method.In the present embodiment, the sheet 31 c used herein is a multilayersheet which includes a UV-curable resin coating over a plastic basesheet. In the method illustrated in FIG. 26( b), the sheet 31 c is putbetween the cylindrical mold 31 and an opposite roller 31 d, and then,the cylindrical mold 31 is rotated. As the cylindrical mold 31 isrotated, the opposite roller 31 d rotates in the direction opposite tothe rotation of the cylindrical mold 31 so that the sheet 31 c isconveyed between the cylindrical mold 31 and the roller 31 d. Then, theUV-curable resin is cured by irradiation with UV light. In this way, thepattern of the recessed and raised portions formed in the side surfaceof the cylindrical mold 31 is transferred to a surface of the sheet 31 cwhich is in contact with the cylindrical mold 31.

On the other hand, when the method illustrated in FIG. 26( c) is used,for example, the distance between a material sheet roll on the feederside and a take-up roll, the rotation speed, etc., are adjusted so thata predetermined pressure is exerted on the sheet 31 c. With suchadjustments, the cylindrical mold 31 is rotated while one surface of themultilayer sheet 31 c which includes a UV-curable resin coating over aplastic base sheet is pressed against the cylindrical mold 31. Thepattern of the cylindrical mold 31 may be transferred while moving thesheet 31 c. The pattern of the cylindrical mold 31 may be transferredwhile moving the cylindrical mold 31. Then, the UV-curable resin iscured by irradiation with UV light. In this way, the pattern of therecessed and raised portions formed in the side surface of thecylindrical mold 31 is transferred to a surface of the sheet 31 c whichis in contact with the cylindrical mold 31.

For example, when a step-and-repeat method is used, discontinuity ofpatterns would be likely to occur due to a positioning error at ajoining part of the mold. However, such discontinuity of patterns wouldnot occur in the present embodiment because the present embodimentemploys a roll-to-roll method, with the use of a cylindrical mold onwhich a linear segment transfer pattern of the minute regions δ isspirally arranged.

FIGS. 29( a) to 29(c) show other arrangement examples of the minuteregions δ over the side surface of the cylindrical mold. In FIG. 29( a),the square minute regions δ (width w×length w) are arranged in twocolumns side by side along the width direction. In each one of thecolumns of this pattern, the minute regions δ₁ (white regions 34 b inthe drawing) and the minute regions δ₂ (dotted regions 34 a in thedrawing) are in a one-dimensionally random arrangement.

Over the side surface of the cylindrical mold 31, the transfer patternas shown in FIG. 29( a) may be spirally arranged. In this case, as shownin FIGS. 29( b) and 29(c), the minute regions δ may be shifted in blocksof two columns. In FIGS. 29( b) and 29(c), among the regions 34W, 34X,34Y, 34Z adjoining along the x′ direction, the regions 34W and 34X areat positions which are coincident in terms of the y′ direction, and theregions 34Y and 34Z are also at positions which are coincident in termsof the y′ direction. However, the region 34X and the region 34Y are atpositions which are different from each other in terms of the y′direction. Basically the same relationship also applies to the otherminute regions δ than the regions 34W, 34X, 34Y, 34Z. Over the sidesurface of the cylindrical mold 31, a transfer pattern formed by threeor more columns of the minute regions δ may be spirally arranged. Inthis case also, the positions of the minute regions δ may be shifted interms of the y′ direction in blocks of three or more columns.

Since the minute regions δ are spirally arranged, the position shift ofthe minute regions δ usually occurs in blocks of a predetermined numberof columns. Therefore, when the minute regions δ of the presentembodiment form a field of m rows by n columns, such a case is unlikelyto occur that n−1 columns are at positions which are coincident in termsof the y′ direction while the remaining one column is at a positionwhich is different from the others in terms of the y′ direction.

(Description of Cylindrical Mold Manufacturing Method)

Examples (1) to (3) of the method of forming the uneven pattern ofrecessed and raised portions in the side surface of the cylindrical mold31 are as follows.

(1) The pattern of the minute regions δ is formed one by one by cuttingthe surface of the cylindrical mold with the use of a bit which issmaller than the minute regions δ (e.g., a diamond arc bit). The cuttingis continued while the mold is rotated and moved till formation of theentire pattern is completed.

(2) The uneven pattern of recessed and raised portions are formed in theside surface of the cylindrical mold 31 by applying a resist over theentire side surface of the cylindrical mold 31, performing a beamexposure process while rotating and moving the mold to form a resistpattern, and then performing an etching process using the resist layeras the mask.

(3) The uneven pattern of recessed and raised portions are formed in theside surface of the cylindrical mold 31 by applying a resist over theside surface of the cylindrical mold 31 by means of inkjet printingwhile rotating and moving the cylindrical mold 31 to form a resistpattern, and then performing an etching process using the resist layeras the mask.

Note that the raised portions formed in the side surface of thecylindrical mold 31 may not have equal heights relative to the referenceplane over the side surface. One piece of the cylindrical mold 31 mayhave raised portions of different heights. Likewise, the recessedportions of the cylindrical mold 31 may not have equal depths relativeto the reference plane over the side surface. One piece of thecylindrical mold 31 may have recessed portions of different depths.

In the present embodiment, the rule for the arrangement of the minuteregions δ₁ and δ₂ along the x′ direction does not matter so long as thearrangement of the minute regions δ₁ and δ₂ along a predeterminedspecific angular direction (the y′ direction along which the minuteregions δ₁ and δ₂ formed in the side surface of the cylindrical mold 31are in a one-dimensionally random arrangement) is random. This holdstrue irrespective of whether minute regions δ adjoining along the x′direction are at positions which are different from one another in termsof the y′ direction, such as the patterns shown in FIGS. 27( c) and27(d) or at positions which are coincident with one another in terms ofthe y′ direction, such as the pattern shown in FIG. 27( b). In thepatterns shown in FIGS. 27( b) to 27(d) and FIGS. 29( b) and 29(c), therandomly arranged minute regions δ are in a spiral arrangement, so thatthe arrangement of the minute regions δ₁ and δ₂ along the x′ directionis also random.

Other Embodiments

The above-described embodiments are merely examples of the presentinvention, and the present invention is not limited to these examples.In the above embodiments, the cross-sectional shape of the raisedportion of the surface structure perpendicular to the surface is notlimited to a rectangle but may be a trapezoid or a circular cone. Thelateral surface of the raised portion may be defined by a curved line.

When the transparent substrate 5 has a large thickness, the point oflight emission occurs at more distant positions from the light radiationpoint S as the number of times of light extraction increases. If thisapplies to a device in which the structure is divided for respectiveones of the pixels of about 300 μm, such as an EL element for displayapplications, light leaks into an adjacent pixel so that the imagequality degrades. One possible solution to this problem is the structureshown in FIG. 30( a) in which the transparent substrate 5 with thesurface structure 13 has a small thickness of about several micrometers,and the transparent substrate 5 is covered with a protection substrate14 of about 0.2 mm to 0.5 mm with an intervening air layer. The frontsurface 14 a and the rear surface 14 b of the protection substrate needan AR coat, although total reflection does not occur on these surfaces.In this case, over the surface structure 13, the air layer may bereplaced by a transparent material of a low refractive index, such as anaerogel or the like. This configuration provides an integral structureand therefore improves the device stability.

In the above embodiments, only one surface has the surface structure 13.However, like structures may be formed on the opposite surfaces of thetransparent substrate 5. A common diffraction grating 13′ may beprovided between the surface structure 13 and the light radiation pointS. In this case, for example, as shown in FIG. 30( b), the transparentsubstrate 5 has a film-like shape, with the surface structure 13 on thefront surface and the diffraction grating 13′ or a surface structure 13″of a modified configuration on the rear surface, and is adhered to thelight emitting body side via an adhesive layer 21. When the refractiveindex of the transparent substrate 5 is small and the difference inrefractive index between the transparent substrate 5 and the lightemitting layer 3 is 0.1 or more, the material of the adhesive layer 21may be selected from materials of refractive indices smaller than thatof the light emitting layer 3 by 0.1 or more, so that total reflectionscarcely occurs at the refracting surface between the adhesive layer 21and the light emitting layer 3. Total reflection which would occur atthe refracting surface between the adhesive layer 21 and the transparentsubstrate 5 and total reflection which would occur at the refractingsurface between the transparent substrate 5 and the air 6 can be avoidedby the surface structure 13″ (or diffraction grating 13′) and thesurface structure 13, respectively. Note that, in the diffractiongrating 13′ and the surface structure 13″, the preferred depth of therecessed portions or the preferred height of the raised portions aredetermined to meet a condition that light transmitted through therecessed portions and light transmitted through the raised portions havea phase difference of n. However, the depth of the recessed portions andthe height of the raised portions may be smaller than those that meetthe above condition.

FIG. 31 is a pattern diagram showing a surface structure for comparison,which has a checker pattern. In FIG. 31, in the surface structure, thesurface of the transparent substrate 5 is divided into squares, eachside having length w, such that gray squares 13 a and white squares 13 bform a checker pattern. The gray squares correspond to the raisedportions, and relatively, the white squares correspond to the recessedportions.

FIG. 32 illustrates the incidence angle dependence of the transmittancet of the surface structure shown in FIG. 31, in which the heightdifference of the raised or recessed portions is d=0.70 μm, under thesame conditions as those of FIG. 19( a). The curves show how much of alight beam whose light amount is 1 in the transparent substrate 5 andwhich is incident on the refracting surface at the incidence angle θ(the angle from the normal to the refracting surface) for the first timeis emitted into the air 6, with the parameter of width w (w=0.1, 0.2,0.4, 1.0, 2.0, 4.0 (μm)). Comparing FIG. 32 with FIG. 16( a) which showsthe characteristics of the random pattern, it can be seen that there aresmall undulations in the curves, except for the curves for w=0.1 μm and0.2 μm (in the range of a so-called nanostructure which does not causediffraction of light). This is because diffracted light appears anddisappears on the air layer side due to diffraction by the checkerpattern. Therefore, this indicates that there is a light intensitydistribution which varies depending on the direction. This is a probleminherent to periodic patterns.

FIG. 19( b) also shows the light extraction efficiencies of thischecker-pattern surface structure and the matrix-lattice surfacestructure shown in FIG. 4( b) (in which the square areas correspond tothe recessed portions) in the first and second light extractions (d=0.70μm, curves 5 e, 5 f, 5E, and 5F). The light extraction efficiency in thesecond light extraction for the matrix-lattice pattern increases as inthe phenomenon described in connection with FIG. 23. This is because theproportion of the raised portions is P=0.75 in the matrix-latticepattern. Both the checker pattern and the matrix-lattice pattern exhibitthe characteristics with undulations which occur depending on thevariation of w as compared with the characteristics of the randompattern. This problem is also inherent to periodic patterns. Thisrelates to a light intensity distribution which varies depending on thedirection.

FIGS. 34( a) and 34(b) also shows the analysis results of the viewingangle dependence of light extracted in the first light extraction, whichwas emitted from the checker-pattern surface structure. Here, the heightdifference is d=0.7 μm, and the boundary width is w=0.5 μm. FIG. 34( a)is under the condition of λ=0.450 μm. FIG. 34( b) is under the conditionof λ=0.635 μm. It can be seen that both solid lines (directions oflongitudes 0° and 90°) and dotted lines (directions of longitudes 45°and 135°) exhibit a large variation relative to the declination, andtheir separation from each other is large. Also, the shape of the lineslargely changes depending on the wavelength. Variation in the lightintensity distribution and color imbalance which occur depending on thedirection are detrimental disadvantages for periodic patters as in thelight emitting device described in Patent Document 1. Thesedisadvantages are all overcome by every one of the first to eightembodiments.

The boundary diffraction effect occurs when portions of light in whichthe phase is discontinuous are separated by a predetermined distance ormore. To maximize this effect, it is necessary to locally maximize theproportion of the phase-discontinuous portions within a restricted area.When the refracting surface is divided into an infinite number of minuteregions such that the phase is discontinuous at the boundary between theminute regions, the aforementioned proportion is locally maximized underthe following two conditions. The first one is that the areas of theminute regions are as equal as possible. The second one is that there isa phase difference between adjacent minute regions. Specifically, if oneof the minute regions is larger than the others, the number ofphase-discontinuous boundaries is increased by dividing this largeminute region. If one of the minute regions is smaller than the others,this means that another one of them is larger than the others, and thenumber of phase-discontinuous boundaries is increased by dividing thislarge minute region. According to an extension of this logic, theproportion of the boundaries between the minute regions can be locallymaximized under the conditions that the areas of the minute regions areas equal as possible, and that each of the areas of the minute regionsis at least from 0.5 times to 1.5 times a predetermined reference area(the diameter of the largest one of the inscribed circles of the minuteregions is from 0.7 times to 1.3 times a predetermined referencediameter). The first to seventh embodiments meet these conditions. Evenwhen the division into the minute regions is locally maximized, theachieved effect decreases if adjacent minute regions have equal phases.Therefore, a phase difference is required between adjacent minuteregions, i.e., random phase allocation is required. The fourth and fifthembodiments meet this condition. In other words, the light emittingdevices of the above embodiments do not realize improvements in theextraction efficiency by the antireflection effect of the light emittingdevice such as described in Patent Document 2, but by the effect oflocal maximization of the boundary diffraction effect.

Note that the surface shapes in the first to eighth embodiments aredifferent from the surface state of frosted glass, surface roughening,or the like, or the surface state of a light emitting device describedin Patent Document 2. In the first, fourth and seventh embodiments, thesurface is divided into square boxes (or polygonal boxes) of apredetermined grid, each box having width w, and raised portions andrecessed portions are allocated to the respective boxes in the ratio of1:1. This pattern has a specific scale, width w, and a specific shape ofthe minute regions, and the ratio of the total area of the raisedportions to the total area of the recessed portions is 1:1. By contrast,the surface state of frosted glass, surface roughening, or the like,does not have a specific width w, and the shape of the minute regions isindefinite. The ratio of the total area of the raised portions to thetotal area of the recessed portions is not 1:1. In the secondembodiment, the ratio of raised portions to recessed portions is shiftedfrom 50%, and accordingly, the ratio of the total area of the raisedportions to the total area of the recessed portions is shifted from 1:1.However, there is still a specific width w, and the ratio between thetotal area of the recessed portions and the total area of the raisedportions has a predetermined value. This is clearly distinguishable fromperfectly random patterns. The third and fifth embodiments also have aspecific width w, and the respective ones of the square boxes (orpolygonal boxes) defined by width w have different heights. Thus, thesurface shapes of the above embodiments do not have a perfectly randompattern but a random pattern which is determined under a predeterminedrule.

The difference of the random patterns of the present embodiments from aperfectly random pattern is further discussed. As shown in FIG. 33( a),eight cards 17 having a width w are randomly arranged on a table 16having a width 4 w. The total area of the eight cards 17 is ½ of thearea of the table 16. Here, it is assumed that none of the cards 17should extend out of the table 16. FIG. 33( b) shows an arrangement inwhich some of the cards 17 are overlapping. FIG. 33( c) shows anarrangement in which none of the cards 17 are overlapping. In FIG. 33(b), the total area of the cards is smaller than ½ of the table area bythe area of overlapping portions of the cards 17. As previouslyillustrated by the curves 27 a and 27A of FIG. 23, the light extractionefficiency decreases as the area ratio deviates from a predeterminedratio. In FIG. 33( c), there is a small gap j between the cards, whichis smaller than w, although the area ratio is maintained at ½. This isalso the case with FIG. 33( b). When the small gap j occurs and thefrequency of its occurrence increases, the gap j can be regarded as anew boundary width. As seen from FIG. 22, the light extractionefficiency greatly decreases under the condition of j<0.2 μm. As seenfrom FIG. 23, as the proportion P₁ of the recessed or raised portions ofthe minute uneven structure increases (the proportion P₁ of thestructure of w₁=0.1 μm increases, such as 0.0, 0.1, and 0.2, in theorder of the curves 27 a, 27 c, and 27 d), both the light extractionefficiencies in the first and second light extractions decrease,although the total proportion of the raised portions is equal. Thus, aperfectly random pattern alone cannot constitute a condition formaximizing the light extraction efficiency.

The principle of random pattern generation employed in the aboveembodiments is different from that of FIG. 33. In the above embodiments,the area ratio is maintained at a predetermined ratio, so that a scalesmaller than the width w, such as the minute gap j, does not occur.Thus, it can be said that the surface shapes of the above embodiments donot have a perfectly random pattern, but a random pattern which isdetermined under a predetermined rule for locally maximizing the lightextraction efficiency.

The phenomenon caused by the surface shapes of the first to eighthembodiments is a form of diffraction phenomenon. As shown in FIG. 5, inthe diffraction phenomenon, a light ray resulting from virtualrefraction relative to a flat reference plane which is equivalent to theaverage of the surface shape is referred to as a zeroth orderdiffraction component (which does not occur in the case of totalreflection), and higher-order diffraction components occur in directionsshifted from the zeroth order diffraction component which is used as areference of the direction. In a random surface shape such as disclosedin the present application, the diffraction components other than thezeroth order occur in random propagation directions. By contrast,frosted glass and surface roughening cause a form of refractionphenomenon, which is different from the diffraction phenomenon. Acrossan uneven refracting surface, the direction of the normal to therefracting surface randomly varies, and accordingly, the direction ofrefraction also randomly varies. For example, when the surface shape ofone of the first to seventh embodiments is formed in one surface of aparallel flat plate, the contour of an image of an object on theopposite side of the plate can be clearly seen through the plate. Thisis because the light diffracted by the surface shape always includes azeroth order diffraction component, and this component serves tomaintain the contour of the image of the object on the opposite side ofthe plate. By contrast, in the case of frosted glass or surfaceroughening, the light does not include a component which is equivalentto the zeroth order diffraction component, so that the contour of theimage of the object on the opposite side appears blurred when seenthrough the plate. Patent Document 2 only describes that light is“obediently emitted into the air” by means of projections formed in thesurface but fails to mention the term “diffraction.” The word“obediently” can be interpreted as “being obedient to Snell's law (lawof refraction).” In this context, it is interpreted as being in the samecategory as frosted glass and surface roughening. Thus, Patent Document2 can be said to be different from the present invention.

The technical feature disclosed in Patent Document 2 is to perfectlyrandomly arrange a plurality of transparent projections over atransparent insulative substrate. Patent Document 2 fails to describe orsuggest the feature of the present invention that recessed and raisedportions are treated as one or more groups of minute regions having thesame shape and the ratio between the raised portions and the recessedportions is set to a specific value. For example, in the firstembodiment, a structure in which the recessed portions and the raisedportions are exchanged or a structure in which the height and depth ofthe respective minute regions are exchanged is substantially the same asthe original structure. This is not the case with the light emittingdevice of Patent Document 2. The present inventors are the first to findthat the features of the illustrative embodiments achieve excellentlight extraction effects, whereas Patent Document 2 fails to describesuch excellent effects as obtained in the above embodiments. In thelight emitting device described in Patent Document 2, projections havinga width of 0.4 μm to 20 μm are in a perfectly random arrangement at adensity of 5000 to 10⁶ projections per unit area (/mm²). Part of thelight emitting devices of the above embodiments fall within the extentof the light emitting device of Patent Document 2 in terms of the formalaspects. However, the relationship between the projections and the otherportions and the relationship of the proportion of the projections, aswell as the effects which cannot achieved without such relationships,are no described or suggested in Patent Document 2. Therefore, the aboveembodiments are not substantially within the scope of the technologiesdisclosed in Patent Document 2. The subject matter disclosed in PatentDocument 2 and the invention of the present application are totallydifferent.

In the first to sixth embodiments and the eighth embodiment, the phaseof light is shifted by an uneven shape consisting of recessed and raisedportions. However, the shift of the phase can be realized by using anyother means than the uneven shape. For example, the shift of the phasecan be realized by determining the thickness and the refractive indexconditions of the multilayer film so as to be different between theregions corresponding to the recessed portions and the regionscorresponding to the raised portions. In this case also, as a matter ofcourse, the same effects as those of the above embodiments can beachieved. Parts of the first to eighth embodiments may be combined intoa new example instead of independently enabling respective one of theseembodiments. Although the first to eighth embodiments have beendescribed with the examples of the organic electroluminescence element,the embodiments are applicable to any element which is capable ofemitting light in a medium whose refractive index is greater than 1. Forexample, the present embodiments are applicable to an LED, a light guideplate, and the like. The medium into which the light emitting deviceemits light is not limited to the air. The surface structures of theabove embodiments are applicable to a transparent substrate whoserefractive index is greater than that of a medium with which thetransparent substrate is in contact, specifically greater than that ofthe medium by 0.1 or more.

INDUSTRIAL APPLICABILITY

As described above, a light emitting device of the present inventiongreatly improves the light extraction efficiency and provides excellentviewing angle characteristics of emitted light, and is therefore usefulfor displays, light sources, etc.

REFERENCE SIGNS LIST

-   -   1 substrate    -   2 electrode    -   3 light emitting layer    -   4 transparent electrode    -   5 transparent substrate    -   6 air    -   13 surface structure    -   30 rotation axis    -   31 cylindrical mold    -   31 a, 31 b region    -   31 c sheet    -   31 d roller    -   32 sheet    -   33 a, 33 b region    -   34 a, 34 b region    -   34 w, 34 x, 34 y, 34 z region    -   S light radiation point

The invention claimed is:
 1. A transparent sheet for use with a lightemitting body with one of surfaces of the transparent sheet beingadjacent to the light emitting body, wherein the other surface of thetransparent sheet includes a plurality of minute regions δ, a largestinscribed circle of the minute regions δ having a diameter from 0.2 μmto 8.0 μm, one of the minute regions δ being adjoined by and surroundedby some other ones of the minute regions δ, the plurality of minuteregions δ include a plurality of minute regions δ₁ which are randomlyselected from the plurality of minute regions δ so as to constitute 40%to 98% of the minute regions δ and a plurality of minute regions δ₂which constitute the remaining portion of the minute regions δ, theminute regions δ₁ are protruding above the other surface to a height ofd/2 relative to a predetermined reference plane parallel to the othersurface, the minute regions δ₂ are receding below the other surface to adepth of d/2 relative to the predetermined reference plane, thepredetermined reference plane is equidistant from the minute regions δ₁and the minute regions δ₂ in terms of a direction perpendicular to theother surface, d is from 0.2 μm to 1.4 μm, the plurality of minuteregions δ have a shape of a polygon in a plane of the other surface, theplurality of minute regions δ are arranged in the plane of the othersurface along a first direction that is parallel to one side of thepolygon, and ones of the plurality of minute regions δ adjoining eachother along the first direction are at positions which are coincidentwith each other in terms of a second direction that is perpendicular tothe first direction, and a pair of minute regions adjoining each otheralong the second direction are at positions which are shifted from eachother in terms of the first direction.
 2. The sheet of claim 1, whereinthe minute regions δ are polygonal and congruent with each other.
 3. Thesheet of claim 1, wherein the plurality of minute regions δ have a shapeof a rectangle or right square in the plane of the other surface, theplurality of minute regions δ are aligned along the first direction thatis parallel to one side of the rectangle or right square, ones of theplurality of minute regions δ adjoining each other along the firstdirection are at positions which are coincident with each other in termsof a second direction that is perpendicular to the first direction, anda pair of minute regions adjoining each other along the second directionare at positions which are shifted from each other in terms of the firstdirection.
 4. The sheet of claim 3, wherein the first direction and thesecond direction are inclined from an edge of the sheet.
 5. A method offabricating the sheet as set forth in claim 1, comprising pressing aside surface of a cylindrical mold against a sheet material such thatrecessed portions and raised portions corresponding to the minuteregions δ are formed in one surface of the sheet material, therebyforming the sheet, wherein the side surface of the cylindrical mold hasrecessed portions and raised portions corresponding to the minuteregions δ, the recessed portions and the raised portions being spirallyarranged around a rotation axis of the cylindrical mold.
 6. The sheet ofclaim 1, wherein the largest inscribed circle of the minute regions δhaving a diameter from 0.2 μm to 1.5 μm.
 7. A transparent sheet for usewith a light emitting body with one of surfaces of the transparent sheetbeing adjacent to the light emitting body, wherein the other surface ofthe transparent sheet includes a plurality of minute regions δ, alargest inscribed circle of the minute regions δ having a diameter from0.2 μm to 8.0 μm, one of the minute regions δ being adjoined by andsurrounded by some other ones of the minute regions δ, respective onesof the plurality of minute regions δ have random heights within a rangeof 0 to d/2 relative to a predetermined reference plane parallel to theother surface or have random depths within a range of 0 to d/2 relativeto the predetermined reference plane, the predetermined reference planeis equidistant from a highest one of the minute regions δ and a lowestone of the minute regions δ in terms of a direction perpendicular to theother surface, d is from 0.2 μm to 1.4 μm, the plurality of minuteregions δ have a shape of a polygon in a plane of the other surface, theplurality of minute regions δ are arranged in the plane of the othersurface along a first direction that is parallel to one side of thepolygon, and ones of the plurality of minute regions δ adjoining eachother along the first direction are at positions which are coincidentwith each other in terms of a second direction that is perpendicular tothe first direction, and a pair of minute regions adjoining each otheralong the second direction are at positions which are shifted from eachother in terms of the first direction.
 8. The sheet of claim 7, whereinthe plurality of minute regions δ have a shape of a rectangle or rightsquare in the plane of the other surface, the plurality of minuteregions δ are aligned along the first direction that is parallel to oneside of the rectangle or right square, ones of the plurality of minuteregions δ adjoining each other along the first direction are atpositions which are coincident with each other in terms of a seconddirection that is perpendicular to the first direction, and a pair ofminute regions adjoining each other along the second direction are atpositions which are shifted from each other in terms of the firstdirection.
 9. The sheet of claim 7, wherein the largest inscribed circleof the minute regions δ having a diameter from 0.2 μm to 1.5 μm.
 10. Atransparent sheet for use with a light emitting body with one ofsurfaces of the transparent sheet being adjacent to the light emittingbody, wherein the other surface of the transparent sheet includes aplurality of minute regions δ, a largest inscribed circle of the minuteregions δ having a diameter from 0.4 μm to 4.0 μm, one of the minuteregions δ being adjoined by and surrounded by some other ones of theminute regions δ, the plurality of minute regions δ include a pluralityof minute regions δ₁ and a plurality of remaining minute regions δ₂, theminute regions δ₁ and the minute regions δ₂ are configured to produce aphase difference of 180° between part of light perpendicularly impingingon the one surface which is transmitted through the minute regions δ₁and another part of the light perpendicularly impinging on the onesurface which is transmitted through the minute regions δ₂, theplurality of minute regions δ have a shape of a polygon in a plane ofthe other surface, the plurality of minute regions δ are arranged in theplane of the other surface along a first direction that is parallel toone side of the polygon, and ones of the plurality of minute regions δadjoining each other along the first direction are at positions whichare coincident with each other in terms of a second direction that isperpendicular to the first direction, and a pair of minute regionsadjoining each other along the second direction are at positions whichare shifted from each other in terms of the first direction.
 11. Thesheet of claim 10, wherein the plurality of minute regions δ have ashape of a rectangle or right square in the plane of the other surface,the plurality of minute regions δ are aligned along the first directionthat is parallel to one side of the rectangle or right square, ones ofthe plurality of minute regions δ adjoining each other along the firstdirection are at positions which are coincident with each other in termsof a second direction that is perpendicular to the first direction, anda pair of minute regions adjoining each other along the second directionare at positions which are shifted from each other in terms of the firstdirection.
 12. The sheet of claim 10, wherein the largest inscribedcircle of the minute regions δ having a diameter from 0.4 μm to 1.0 μm.13. A light emitting device, comprising a light emitting body and atransparent protection layer provided on a light emitting surface of thelight emitting body, wherein the transparent protection layer has asurface which adjoins the light emitting surface and a surface oppositeto the adjoining surface, the opposite surface including a plurality ofminute regions δ, a largest inscribed circle of the minute regionsδhaving a diameter from 0.2 μm to 8.0 μm, one of the minute regions δbeing adjoined by and surrounded by some other ones of the minuteregions δ, the plurality of minute regions δ include a plurality ofminute regions δ₁ which are randomly selected from the plurality ofminute regions δ so as to constitute 40% to 98% of the minute regions δand a plurality of minute regions δ₂ which constitute the remainingportion of the minute regions δ, the minute regions δ₁ are protrudingabove the other surface to a height of d/2 relative to a predeterminedreference plane parallel to the other surface, the minute regions δ₂ arereceding below the other surface to a depth of d/2 relative to thepredetermined reference plane, the predetermined reference plane isequidistant from the minute regions δ₁ and the minute regions δ₂ interms of a direction perpendicular to the other surface, the lightemitting body is configured to emit light whose center wavelength of anemission spectrum is λ, λ/6(n₁−n₀)<d<λ/(n₁−n₀) holds where n₁ is arefractive index of the protection layer and n₀ is a refractive index ofa medium with which the protection layer is in contact at the oppositesurface, n₀ being smaller than n₁, the plurality of minute regions δhave a shape of a polygon in a plane of the other surface, the pluralityof minute regions δ are arranged in the plane of the other surface alonga first direction that is parallel to one side of the polygon, and onesof the plurality of minute regions δ adjoining each other along thefirst direction are at positions which are coincident with each other interms of a second direction that is perpendicular to the firstdirection, and a pair of minute regions adjoining each other along thesecond direction are at positions which are shifted from each other interms of the first direction.
 14. The light emitting device of claim 13,wherein the medium is air.
 15. The light emitting device of claim 13,wherein the medium is aerogel.
 16. The light emitting device of claim13, wherein n₂−n₁<0.1 holds where n₂ is a refractive index of part ofthe light emitting body from which light is radiated.
 17. The lightemitting device of claim 13, wherein the plurality of minute regions δhave a shape of a rectangle or right square in the plane of the othersurface, the plurality of minute regions δ are aligned along the firstdirection that is parallel to one side of the rectangle or right square,ones of the plurality of minute regions δ adjoining each other along thefirst direction are at positions which are coincident with each other interms of a second direction that is perpendicular to the firstdirection, and a pair of minute regions adjoining each other along thesecond direction are at positions which are shifted from each other interms of the first direction.
 18. A method of fabricating the lightemitting device as set forth in claim 13, comprising the steps of:pressing a side surface of a cylindrical mold against a protection layermaterial such that recessed portions and raised portions correspondingto the minute regions δ are formed in one surface of the protectionlayer material, thereby forming the protection layer; and placing theprotection layer on an emission surface of the light emitting body suchthat a surface of the protection layer opposite to the one surface iscloser to the light emitting body, wherein the side surface of thecylindrical mold has recessed portions and raised portions correspondingto the minute regions δ, the recessed portions and the raised portionsare spirally arranged around a rotation axis of the cylindrical mold.19. The light emitting device of claim 13, wherein the largest inscribedcircle of the minute regions δ having a diameter from 0.2 μm to 1.5 μm.20. A light emitting device, comprising a light emitting body and atransparent protection layer provided on a light emitting surface of thelight emitting body, wherein the transparent protection layer has asurface which adjoins the light emitting surface and a surface oppositeto the adjoining surface, the opposite surface including a plurality ofminute regions δ, a largest inscribed circle of the minute regions δhaving a diameter from 0.2 μm to 8.0 μm, one of the minute regions δbeing adjoined by and surrounded by some other ones of the minuteregions δ, respective ones of the plurality of minute regions δ haverandom heights within a range of 0 to d/2 relative to a predeterminedreference plane parallel to the other surface or have random depthswithin a range of 0 to d/2 relative to the predetermined referenceplane, the predetermined reference plane is equidistant from a highestone of the minute regions δ and a lowest one of the minute regions δ interms of a direction perpendicular to the other surface, the lightemitting body is configured to emit light whose center wavelength of anemission spectrum is λ, λ/6(n₁−n₀)>d>λ/(n₁−n₀) holds where n₁ is arefractive index of the protection layer and n₀ is a refractive index ofa medium with which the protection layer is in contact at the oppositesurface, n₀ being smaller than n₁, and the plurality of minute regions δhave a shape of a polygon in a plane of the other surface, the pluralityof minute regions δ are arranged in the plane of the other surface alonga first direction that is parallel to one side of the polygon, and onesof the plurality of minute regions δ adjoining each other along thefirst direction are at positions which are coincident with each other interms of a second direction that is perpendicular to the firstdirection, and a pair of minute regions adjoining each other along thesecond direction are at positions which are shifted from each other interms of the first direction.
 21. The light emitting device of claim 20,wherein the plurality of minute regions δ have a shape of a rectangle orright square in the plane of the other surface, the plurality of minuteregions δ are aligned along the first direction that is parallel to oneside of the rectangle or right square, ones of the plurality of minuteregions δ adjoining each other along the first direction are atpositions which are coincident with each other in terms of a seconddirection that is perpendicular to the first direction, and a pair ofminute regions adjoining each other along the second direction are atpositions which are shifted from each other in terms of the firstdirection.
 22. The light emitting device of claim 20, wherein thelargest inscribed circle of the minute regions δ having a diameter from0.2 μm to 1.5 μm.
 23. A light emitting device, comprising a lightemitting body and a transparent protection layer provided on a lightemitting surface of the light emitting body, wherein the transparentprotection layer has a surface which adjoins the light emitting surfaceand a surface opposite to the adjoining surface, the opposite surfaceincluding a plurality of minute regions δ, a largest inscribed circle ofthe minute regions δ having a diameter from 0.4 μm to 4.0 μm, one of theminute regions δ being adjoined by and surrounded by some other ones ofthe minute regions δ, the plurality of minute regions δ include aplurality of minute regions δ₁ and a plurality of remaining minuteregions δ₂, the minute regions δ₁ and the minute regions δ₂ areconfigured to produce a phase difference of 180° between part of lightperpendicularly impinging on the one surface which is transmittedthrough the minute regions δ₁ and another part of the lightperpendicularly impinging on the one surface which is transmittedthrough the minute regions δ₂, and the plurality of minute regions δhave a shape of a polygon in a plane of the other surface, the pluralityof minute regions δ are arranged in the plane of the other surface alonga first direction that is parallel to one side of the polygon, and onesof the plurality of minute regions δ adjoining each other along thefirst direction are at positions which are coincident with each other interms of a second direction that is perpendicular to the firstdirection, and a pair of minute regions adjoining each other along thesecond direction are at positions which are shifted from each other interms of the first direction.
 24. The light emitting device of claim 23,wherein the plurality of minute regions δ have a shape of a rectangle orright square in the plane of the other surface, the plurality of minuteregions δ are aligned along the first direction that is parallel to oneside of the rectangle or right square, ones of the plurality of minuteregions δ adjoining each other along the first direction are atpositions which are coincident with each other in terms of a seconddirection that is perpendicular to the first direction, and a pair ofminute regions adjoining each other along the second direction are atpositions which are shifted from each other in terms of the firstdirection.
 25. The light emitting device of claim 23, wherein a largestinscribed circle of the minute regions δ having a diameter from 0.4 μmto 1.0 μm.